Cancer Vaccines and Tumor Immunity

CANCER VACCINES AND TUMOR IMMUNITY Rimas Orentas, PhD Department of Pediatrics Medical College of Wisconsin James W. ...

Author: Rimas Orentas | James W. Hodge | Bryon D. | PhD Johnson

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CANCER VACCINES AND TUMOR IMMUNITY

Rimas Orentas, PhD Department of Pediatrics Medical College of Wisconsin

James W. Hodge, PhD Laboratory of Tumor Immunology and Biology National Cancer Isntitute

Bryon D. Johnson, PhD Department of Pediatrics–Hematology–Oncology Medical College of Wisconsin

A JOHN WILEY & SONS, INC., PUBLICATION



Rimas Orentas, PhD Department of Pediatrics Medical College of Wisconsin

James W. Hodge, PhD Laboratory of Tumor Immunology and Biology National Cancer Isntitute

Bryon D. Johnson, PhD Department of Pediatrics–Hematology–Oncology Medical College of Wisconsin

A JOHN WILEY & SONS, INC., PUBLICATION

Copyright © 2008 by John Wiley & Sons, Inc. All rights reserved. Published by John Wiley & Sons, Inc., Hoboken, New Jersey Published simultaneously in Canada No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permission. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com. Library of Congress Cataloging-in-Publication Data: Cancer vaccines and tumor immunity / [edited by] Rimas Orentas, James W. Hodge, Bryon D. Johnson. p. ; cm. Includes bibliographical references and index. ISBN 978-0-470-07474-9 (cloth) 1. Cancer vaccines. 2. Tumors—Immunological aspects. I. Orentas, Rimas. II. Hodge, James W. III. Johnson, Bryon D. [DNLM: 1. Neoplasms—therapy. 2. Cancer Vaccines—therapeutic use. 3. Drug Design. 4. Immunotherapy, Active. 5. Neoplasms—immunology. QZ 266 C215 2008] RC271.I45C38 2008 616.99 407—dc22 2007006010 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

CONTENTS

Contributors Foreword CANCER VACCINES AND CANCER IMMUNOTHERAPY: A NEW PARADIGM Jeffrey Schlom

I

CANCER VACCINES: PROGRESS AND PROMISE Rimas J. Orentas, Bryon Johnson, and James Hodge

3

ADJUVANT THERAPY: ENHANCING THE ENDOGENOUS IMMUNE RESPONSE

2 3 4

III

xiii

INTRODUCTION

1

II

ix

FULLY SYNTHETIC CARBOYHYDRATE-BASED ANTITUMOR VACCINES Rebecca M. Wilson and Samuel J. Danishefsky

15

´ BACILLUS CALMETTE–GUERIN IMMUNOTHERAPY OF GENITOURINARY CANCER Donald L. Lamm

29

STIMULATION OF TOLL-LIKE RECEPTOR 9 FOR ENHANCING VACCINATION Daniel E. Speiser and Arthur M. Krieg

43

ANTIGEN-SPECIFIC THERAPY: NOVEL PRESENTATION OF PEPTIDE AND PROTEIN ANTIGENS

5

POLYEPITOPE VACCINES Corey Smith and Rajiv Khanna

69

v

vi

CONTENTS

6

7 8

9

IV

ANTIGEN-SPECIFIC CANCER IMMUNOTHERAPY: HPV-ASSOCIATED CERVICAL CANCER AS A MODEL SYSTEM Shaw-Wei D. Tsen, Chien-Fu Hung, and T.-C. Wu POXVIRAL VECTORS FOR CANCER VACCINES: STATE OF THE ART Elizabeth K. Wansley IMMUNOTHERAPEUTIC STRATEGIES AGAINST CANCER USING Listeria monocytogenes AS A VECTOR FOR TUMOR ANTIGENS Nicholas C. Souders, Thorsten Verch, and Yvonne Paterson COUPLING INNATE AND ADAPTIVE IMMUNITY WITH YEAST-BASED CANCER IMMUNOTHERAPY Sibyl Munson, Joanne Parker, Thomas H. King, Yingnian Lu, Victoria Kelley, Zhimin Guo, Virginia Borges, and Alex Franzusoff

79

95

113

131

CELL-BASED THERAPY: USING CANCER CELLS AS A MEANS TO INDUCE SPECIFIC TUMOR IMMUNITY

10

ALLOGENEIC WHOLE-CELL VACCINES John Copier and Angus Dalgleish

11

JUMP-STARTING TUMOR IMMUNITY WITH BREAST CANCER THERAPEUTICS Leisha A. Emens

12

13

14

T-REGULATORY CELL MANIPULATION IN TUMOR IMMUNOTHERAPY Jens Rüter, Brian G. Barnett, Ilona Kryczek, Michael J. Brumlik, Benjamin J. Daniel, George Coukos, Weiping Zou, and Tyler J. Curiel TUMOR VACCINATION AFTER AUTOLOGOUS HSCT: WHAT HAS BEEN LEARNED FROM EXPERIMENTAL MODELS Weiqing Jing and Bryon D. Johnson VACCINES IN THE SETTING OF HEMATOPOIETIC STEM CELL TRANSPLANTATION Ronald Gress and Claude Sportes

153

169

189

205

217

CONTENTS

15 16

V

CANCER IMMUNOTHERAPY: UNTAPPING THE POTENTIAL OF COSTIMULATORY MOLECULES BEYOND CTLA-4 Mathew Augustine and Lieping Chen

229

243

DEFINING EFFECTIVE CLINICAL RESPONSES

17

18 Index

INTRATUMOR GENERATION OF VIGOROUS ANTITUMOR IMMUNE RESPONSES Ping Yu and Yang-Xin Fu

vii

ADVANCES IN IMMUNE MONITORING STRATEGIES FOR CANCER VACCINES AND IMMUNOTHERAPY Seunghee Kim-Schulze, Moshe Ornstein, and Howard L. Kaufman IMMUNE EVALUATION OF CANCER VACCINES Theresa L. Whiteside

271

291 309

CONTRIBUTORS

Mathew Augustine, MD Departments of Immunology and Surgery, The Johns Hopkins School of Medicine The Johns Hopkins Hospital, Baltimore, MD Brian G. Barnett, MD Tulane Medical School, New Orleans, LA Virginia Borges, MD Department of Medicine, University of Colorado Health Sciences Center, Aurora, CO Michael J. Brumlik, PhD The University of Texas Health Science Center at San Antonio, San Antonio, TX Lieping Chen, MD, PhD Departments of Dermatology, Oncology, and Immunology, and The Institute for Cell Engineering, The Johns Hopkins University School of Medicine, Baltimore, MD John Copier, PhD Oncology/Cellular and Molecular Medicine, St. George’s University of London, London, UK George Coukos, MD, PhD University of Pennsylvania, Philadelphia, PA Tyler J. Curiel, MD, MPH Director, San Antonio Cancer Institute, The University of Texas Health Science Center at San Antonio, San Antonio, TX, and Scientific Director, Cancer Therapy and Research Center, San Antonio, San Antonio, TX Angus Dalgleish, Bsc (Hons), MD, FRACPath, FRACP, FRCP, FMedSci Oncology/Cellular and Molecular Medicine, St. George’s University of London, London, UK Benjamin J. Daniel, PhD The University of Texas Health Science Center at San Antonio, San Antonio, TX Samuel J. Danishefsky, PhD Kettering Chair and Director, Laboratory for Bioorganic Chemistry, Sloan-Kettering Institute, and Professor of Chemistry, Columbia University, New York, NY Leisha A. Emens, MD, PhD Department of Oncology, Johns Hopkins University School of Medicine, Baltimore, MD Alex Franzusoff, PhD V.P. R&D and Founding Scientist, GlobeImmune, Louisville, CO Yang-Xin Fu, MD PhD Department of Pathology, Committee for Immunology, Committee for Molecular Pathogenesis and Molecular Medicine, University of Chicago, Chicago, IL ix

x

CONTRIBUTORS

Ronald Gress, MD Chief, Experimental Transplantation and Immunology Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD Zhimin Guo, PhD GlobeImmune, Louisville, CO James Hodge, PhD Department of Medical Biology, University of Tennessee Medical Center and Laboratory of Tumor Immunology and Biology, National Cancer Institute, National Institutes of Health, Bethesda, MD Chien-Fu Hung Department of Pathology, The Johns Hopkins Medical Institutions, Baltimore, MD Weiqing Jing, MD Department of Pediatrics, Medical College of Wisconsin and the Children’s Research Institute, Children’s Hospital of Wisconsin. Milwaukee, WI Bryon Johnson, PhD Department of Pediatrics, Medical College of Wisconsin and the Children’s Research Institute, Children’s Hospital of Wisconsin. Milwaukee, WI Howard L. Kaufman, MD Director, Tumor Immunology Laboratory, Division of Surgical Oncology, Columbia University Medical Center, New York NY Victoria Kelley, PhD GlobeImmune, Louisville, CO Rajiv Khanna, PhD Australian Centre for Vaccine Development, Tumour Immunology Laboratory, Principal Research Fellow (NH&MRC) Division of Infectious Diseases and Immunology, Queensland Institute of Medical Research, Herston, QLD, Australia Seunghee Kim-Schulze, PhD The Tumor Immunology Laboratory, Division of Surgical Oncology, Columbia University Medical Center, New York, NY Tom King, PhD GlobeImmune, Louisville, CO Arthur M. Krieg, MD Department of Internal Medicine, The University of Iowa, Iowa City, IA, and Coley Pharmaceutical Group, Wellesley, MA Ilona Kryczek, PhD The University of Michigan, Ann Arbor, MI Donald L. Lamm, MD Clincial Professor, University of Arizona and President, BCG Oncology, Phoenix, AZ Yingnian Lu, MD GlobeImmune, Louisville, CO Sibyl Munson, PhD GlobeImmune, Louisville, CO Rimas J. Orentas, PhD Departments of Pediatrics, Medicine, Microbiology and Molecular Genetics, Medical College of Wisconsin and the Children’s Research Institute, Children’s Hospital of Wisconsin, Milwaukee, WI Moshe Ornstein, BS The Tumor Immunology Laboratory, Division of Surgical Oncology, Columbia University Medical Center, New York NY Joanne Parker, PhD GlobeImmune, Louisville, CO Yvonne Paterson, PhD Department of Microbiology, University of Pennsylvania, Philadelphia, PA

CONTRIBUTORS

xi

Jens Rueter, MD The University of Texas Health Science Center at San Antonio, San Antonio, TX Jeffrey Schlom, PhD Chief, Laboratory of Tumor Immunology and Biology Center for Cancer Research, National Cancer Institute, National Institute of Health, Bethesda, MD Corey Smith, PhD Australian Centre for Vaccine Development, Tumour Immunology Laboratory, Division of Infectious Diseases and Immunology, Queensland Institute of Medical Research, Herston, QLD, Australia Nicholas C. Souders, PhD Department of Microbiology, University of Pennsylvania, Philadelphia, PA Daniel E. Speiser, MD Division of Clinical Onco-Immunology, Ludwig Institute for Cancer Research, University Hospital of Lausanne, Switzerland Claude Sportes, MD Clinical Research Core, Experimental Transplantation and Immunology Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD Shaw-Wei D. Tsen Department of Pathology, The Johns Hopkins Medical Institutions, Baltimore, MD Thorsten Verch, PhD Department of Microbiology, University of Pennsylvania, Philadelphia, PA Elizabeth K. Wansley, PhD Laboratory of Tumor Immunology and Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD Theresa L. Whiteside, PhD Director of Immunologic Monitoring and Cellular Products Laboratory (IMCPL), University of Pittsburgh Cancer Institute and Department of Pathology, University of Pittsburgh School of Medicine, Pittsburgh, PA Rebecca M. Wilson, PhD Bioorganic Chemistry Laboratory, Memorial SloanKettering Cancer Center, Columbia University, New York, NY T.C. Wu, MD, PhD Departments of Pathology, Oncology, Obstetrics and Gynecology and Molecular Microbiology and Immunology, The Johns Hopkins Medical Institutions, Baltimore, MD Ping Yu, MD Section of Dermatology, Department of Medicine, University of Chicago, Chicago, IL Weiping Zou, MD, PhD The University of Michigan, Ann Arbor, MI

FOREWORD CANCER VACCINES AND CANCER IMMUNOTHERAPY: NEW PARADIGMS Jeffrey Schlom

The field of cancer vaccines is currently in an active state of both preclinical and clinical investigation. Hypothesis-driven preclinical studies in both in vivo rodent models and employing in vitro human systems are readily being translated into science-driven clinical trials. The basic concept in the use of vaccines for cancer therapy is to define strategies in which the immune system recognizes tumor-associated antigens that are not otherwise being recognized in the tumor-bearing host. There are numerous components of the immune system that vaccines have been shown to activate. These include cytolytic CD8 T cells, helper CD4 T cells, antibodies, natural killer (NK) cells, and other components of the innate immune system. Indeed, recent (as of 2007) studies are demonstrating a more intimate association than had been previously thought between the innate and the adaptive immune systems in cancer immunity. Equally important are the findings that a range of suppressor cells, such as regulatory T cells, immature macrophages, and other immunosuppressive entities and signals must be dealt with for the effective generation of an immune response and antitumor immunity. Both classical chemotherapeutic drugs, such as cyclophosphamide, and monoclonal antibodies directed against immune suppressive elements such as CTLA4 (cytotoxic T lymphocyte antigen 4), are examples of strategies that are actively being investigated. A major aspect of this book is the elucidation of the wide breadth of vaccine vehicles and strategies that are being employed to enhance the activation of the host immune system to tumor-associated antigens. Anticancer vaccines that are being investigated include the use of recombinant viral vectors, recombinant bacterial vectors, peptides and polypeptides, carbohydrates, proteins, whole tumor cells, and xiii

xiv

CANCER VACCINES AND CANCER IMMUNOTHERAPY: NEW PARADIGMS

yeast, to deliver antigens to dendritic cells and other antigen-presenting cells. The area of immunostimulants to enhance vaccine efficacy is an equally important field of research. Strategies discussed in these chapters include the use of T cell costimulatory molecules in vectors, and biological adjuvants to activate the immune system, such as cytokines, CpG motifs, and bacillus Calmette–Guérin (BCG). One issue frequently raised in the use of cancer vaccines is antigenic heterogeneity of tumors and tumor cell antigen escape variants. More recent clinical studies employing whole tumor cell vaccines and viral vector-based vaccines have provided clinical evidence of the phenomenon of “antigen cascade” or “epitope spreading” postvaccination. This well-studied phenomenon in preclinical models has been shown to be the result of cross-presentation of tumor antigens to antigen-presenting cells as a consequence of some tumor cell destruction postvaccination. Consequently, the host immune system is exposed to tumor antigens other than those in the vaccine. The analysis of the clinical activity of therapeutic cancer vaccines, as is the case with other new targeted therapies, may well require a new paradigm. The classical paradigm for cancer therapeutics has been to first demonstrate reduction in the size of measurable tumor masses by the established RECIST criteria (response evaluation criteria in solid tumors, which looks for a greater than 30% reduction in tumor volume; further details available at http://ctep.cancer.gov/guidelines/ recist.html). Often, however, these reductions in tumor size are concurrent with substantial toxicity to the patient, and in some cases with minimal or no demonstrated increase in patient survival. For example, a randomized clinical study comparing higher vs. lower doses of interleukin 2 (IL-2) showed more tumor regressions in patients with metastatic renal cell cancer employing the higher dose of IL-2, but no statistical increase in patient survival when compared with the lower dose. Similarly, while adoptive transfer of T cells has been shown to dramatically reduce tumor burden in subsets of melanoma patients, to date, no randomized trial has demonstrated an advantage in patient survival using this approach as compared to the use of IL-2 alone. Cancer vaccines, on the other hand, induce a dynamic process that may or may not result in the substantial reduction of tumor masses by RECIST criteria, but may result in tumor stasis, low levels of toxicity, and most importantly, increased patient survival. For example, this paradigm has recently been demonstrated in small-molecule-targeted therapies such as the use of sorafenib. In a randomized phase III trial involving 903 patients with metastatic renal cell carcinoma, treatment with sorafenib resulted in only 1/451 complete responses, and 3-fold increase in PSA-specific T cells

Prostate

I

26

Vaccinia (rV-PSA)

Prostate

II

Vaccinia (rV-TRICOM) Vaccinia (rF-PSA and rF-B7-1) Vaccinia (rV-B7-1)

Melanoma

I

Prostate

II

Melanoma

I

Vaccinia

Bladder

I

Vaccinia (rV-PSA) + / − GM-CSF Vaccinia (rV-GMCSF) Fowlpox-gp100

Prostate

I

Melanoma

I

Melanoma

II

MVA-hTyr

Melanoma

I

MVA

Melanoma

I

MVA (TG4010)

MUC1-(+) solid tumors

I

MVA-5 T4 (TroVax)

Colorectal

Minimal toxicity; 1 patient showed very low levels of PSA ( < 0.2 ng/mL) for > 8 months 78.1% clinical progression free survival; 46% of patients showed an increase in PSA-specific T cells 30.7% objective response rate; 1 complete response for > 22 months Median progression free survival with vaccine and docetaxel was 6.1 months as compared to docetaxel alone 1 of 12 patients showed a partial response; 2 patients showed stable disease; increased frequency of melanoma antigen-specific T cells 3 of 4 patients showed significant immune cell infiltration; these 3 patients were disease-free 4 years posttreatment 14 of 33 patients showed stable disease for at least 6 months; patients showed a PSA-specific T cell response 1 of 7 patients showed a partial response; 1 patient had a complete response 10 of 14 patients showed reactivity to gp100 Strong response to viral vector, but no measurable immune response to hTyr 4 of 7 patients generated CTL specific for a high-affinity melanoma-specific epitope expressed in the vaccine 4 of 13 patients showed stable disease for 6–9 months; 5 patients showed T cell immune responses 16 of 17 patients showed 5 T4-specific cellular immune responses; 5 patients showed stable disease from 3 to 18 months; positive correlation between 5 T4 antibody response and patient survival or time to progression

Vaccinia (rV-PSA)

I/II

42

70

71 72

73

74

75

50 76 77 78

79

61

100

POXVIRAL VECTORS FOR CANCER VACCINES: STATE OF THE ART

Diversified Prime and Boost To be effective, a vaccine must be administered multiple times in order to sufficiently prime the immune response against the desired antigen. The first vaccine (the prime) should be able to induce a strong T cell response against the antigen, and additional vaccinations (boosts) should further increase the T cell response to the antigen [17]. However, if the same vector is used for the prime and subsequent boosts, the immune response against the vector may prohibit the boosts from being effective. Evidence of this was seen in the clinical trial mentioned above, in which there was no increase in CEA-specific immune responses by the third vaccination of rV-CEA [24,25]. Fowlpox (avipox) virus is an ideal choice for a boost following a vaccinia prime. Although fowlpox is replication-defective in mammalian cells, it can infect these cells and express the transgenes for 14–21 days before the cell dies [28]. In a preclinical study, it was shown that CEA-specific T cells were induced 4 times more when vaccinating with rV-CEA as a prime and boosting with avipox-CEA than when avipox-CEA was given alone 3 times [29]. To determine which prime–boost regimen to use, a clinical trial was conducted in which patients were given either rV-CEA as a prime followed by three avipox-CEA boosts (VAAA) or three avipox-CEA vaccinations followed by an rV-CEA boost (AAAV). Antigen-specific immune responses were better in the VAAA group than the AAAV group (Table 7.1) [30]. Additionally, on further follow-up, five of nine patients in the VAAA arm were still alive, while zero of nine patients were still alive in the AAAV arm; these results correlated with immune responses observed [31]. TRICOM Vectors Induction of an immune response requires both antigen presentation in a peptide– MHC complex on an antigen-presenting cell (APC; signal 1) and T cell costimulatory molecules on the surface of the APC (signal 2) [22,32,33]. Because TAAs are normally self-antigens that are overexpressed on tumor cells, they are generally considered weak antigens, as the immune system does not respond well to an antigen that it perceives as self [34]. In the presence of a weak signal 1 such as a TAA, T cell costimulation is especially important [35]. It has been hypothesized that tumor cells can evade the immune system by not expressing sufficiently high levels of costimulatory molecules [36–38]. As such, one focus of cancer vaccines is to introduce costimulatory molecules into cancer vaccines along with TAAs to increase antitumor immune responses. B7-1 is one of the most widely studied costimulatory molecules, and preclinical studies have shown that the addition of B7-1 can increase the immunogenicity of a weakly immunogenic tumor [39–41]. Several phase II clinical trials have been conducted using an admixture of recombinant vaccinia virus vectors expressing the TAA prostate-specific antigen and B7-1 (rV-PSA and rV-B7-1, respectively) for the treatment of prostate cancer [42–44]. These trials demonstrated the safety of using these vectors in combination with radiotherapy, hormone therapy, or chemotherapy, and suggest improved clinical benefit when given with the vaccinia vectors over the respective therapy alone.

USING VACCINIA VECTORS TO EXPRESS CYTOKINES

101

Two other important costimulatory molecules are intercellular adhesion molecule 1 (ICAM-1) and leukocyte function-associated antigen 3 (LFA-3), both of which are expressed on APCs and are involved in adhesion between the APC and T cell, thereby increasing their interaction time [22]. A recombinant vaccinia virus expressing B7-1, ICAM-1, and LFA-3 was created (designated TRICOM for tri ad of costimulatory molecules), in which each molecule primes a unique signaling pathway in T cells [45,46]. In vitro studies comparing T cell activation by cells infected with vectors containing one or all three costimulatory molecules showed that vectors containing all three molecules activated T cells to a much higher degree than that seen with vectors containing any of the molecules alone [46]. When mice were vaccinated with vectors containing CEA and either B7-1 alone (rF-CEA/B7) or TRICOM (rF-CEA/TRICOM), it was found that just one vaccination with rF-CEA/TRICOM elicited a CEA-specific T cell response that was as great as four vaccinations with rF-CEA/B7. Additional vaccinations with rF-CEA/TRICOM further increased the antigen-specific T cell response in mice [47]. The first clinical trial using TRICOM was undertaken in patients with CEA-positive malignancies, the majority of which were colorectal or gastrointestinal (GI) tract cancers (Table 7.1) [34,48]. Patients were vaccinated with a recombinant fowlpox virus expressing an agonist epitope of CEA along with the three costimulatory molecules (rF-CEA(6D)-TRICOM); once safety was established, subsequent patients were vaccinated with an rV-CEA(6D)-TRICOM prime and monthly rF-CEA(6D)-TRICOM boosts. No maximum tolerated dose was reached, and toxicity was minimal. Of the 58 patients enrolled in the trial, 11 showed stable disease initially. Of 25 patients who initially progressed chose to continue therapy through four cycles, 12 showed stable disease, having benefited from the vaccine. After these 12 patients were moved from monthly vaccinations to once every 3 months, all 12 progressed. Interestingly, when 11 of these 12 patients were moved back to the once per month schedule, 6 then restabilized, suggesting that for the vaccinations to be effective, they needed to be given monthly. One patient showed a pathologic complete response. Additionally, in the majority of patients, enhanced CEA-specific T cell responses were observed [48]. The results from this first phase I trial were promising and paved the way for additional phase I/II trials with TRICOM vectors.

7.4

USING VACCINIA VECTORS TO EXPRESS CYTOKINES

Cytokines such as granulocyte macrophage colony-stimulating factor (GM-CSF) and interleukin 2 (IL-2) have been used in the immunotherapy of cancer, usually administered as purified proteins. However, it has been demonstrated that another effective method of delivery of these cytokines is through expression by poxvirus vectors. In two separate phase I trials, a recombinant vaccinia virus expressing IL-2 along with the tumor antigen MUC1 was administered to patients. In one of these trials, 2 of 28 patients showed a partial response, while 15 patients exhibited stable disease (Table 7.1) [49].

102


100 90 80

% Survival

70 60 50

Cohorts 7 & 8 VAAA + GM-CSF

40 30 20

Cohorts 3 & 6 AAAA

10 0 0

6

12 18 Survival Time (Months)

24

30

Figure 7.1. Overall survival by cohorts with or without granulocyte–macrophage colony-stimulating factor (GM-CSF). Cohorts 3 and 6 (16 of 19 patients experienced treatment failure) received no Gm-CSF with vaccines; cohorts 7 and 8 (12 of 24 patients experienced treatment failure) received vaccines plus GM-CSF. (Taken from Ref. 48.)

When a recombinant vaccinia virus expressing GM-CSF was given to melanoma patients in a phase I trial, evidence of tumor regression (with one complete remission) was observed (Table 7.1) [50]. When GM-CSF was given along with rV-CEA/ TRICOM, it was observed that patients who received GM-CSF along with vaccine showed a longer progression-free survival than did those who received vaccine alone (Fig. 7.1) [48]. Because of these promising results, rF-GM-CSF has been incorporated into the vaccine regimen in current and future trials with TRICOM vectors.

7.5

COMBINATION THERAPIES

Radiation and chemotherapy are the standard of care for many types of cancer. As such, research groups have begun to combine vaccines with one of these treatment modalities in an effort to increase vaccine efficacy. In vitro studies have shown that exposing tumor cells to sublethal doses of radiation alters the phenotype of these cells by upregulating a number of genes such as Fas, MHC class I, ICAM-1, and the TAAs CEA and MUC1 [51–53]. As a consequence, these tumor cells become more susceptible to T-cell-mediated killing. Local irradiation of tumors in mice after vaccination with rV-CEA/TRICOM showed a synergistic increase in antitumor efficacy as compared with either modality alone, as no mice were

COMBINATION THERAPIES

No Treatment

500 n = 12

Irradiation 11 (41)

(b) n = 13

Vaccine + Irradiation 66 (435)

(c) n = 12

54 (415)

(d)

CEA Positive Tumors

400

Vaccine

12 (36)

(a)

n=9

200 100

400

n=0 7 (16)

(e) n = 10

n=0 6 (21)

(f ) n = 10

(g)

n=0 64 (593)

n=9

n=5 68 (654)

(h)

CEA Negative Tumors

Tumor Volume (mm3)

300

0 500

n=9

300 200 100 0

n=0 0

7 14 21 28 35 42 49 0

n=0 7 14 21 28 35 42 49 0

103

n=0 7 14 21 28 35 42 49 0

n=0 7 14 21 28 35 42 49

Days Post Tumor Transplant

Figure 7.2. The Vaccination regimen consists of poxviral vectors that express a triad of costimulatory molecules called TRICOM (B7-1, ICAM-1, LFA-3), in addition to the tumor-associated antigen CEA. Although either modality alone was ineffective, the combination resulted in tumlor cures in 50% of the mice bearing CEA+ tumors. This resulted further in protection from subsequent tumor challenge. Interestingly, none of the mice bearing CEA tumors were cured of their tumor masses suggesting that an antigen-specific immune response mechanism was responsible. (Taken from Ref. 22.)

cured after either radiation or vaccine alone, but 50% of mice given both therapies were cured of their tumors (Fig. 7.2) [52]. A phase II clinical trial involving the combination of vaccine and radiotherapy was conducted in patients with localized prostate cancer (Table 7.1) [42]. Patients in the vaccine–radiotherapy arm were given a combination of rV-PSA and rV-B7-1 as a prime and rF-PSA boosts monthly along with local GM-CSF and low-dose interleukin-2 (IL-2). External beam radiation was given between the fourth and sixth vaccinations. Of 17 patients in the vaccine–radiotherapy arm, 13 showed a significant increase in the number of PSA-specific T cells (at least threefold) after therapy, while patients in the radiotherapy-only arm showed no detectable increase in PSA-specific T cells. Additionally, it was found that T cells were generated to other prostate-associated antigens not found in the vaccine, suggesting immune-mediated killing of tumor cells. These results have led to an additional clinical trial using vaccine and radiotherapy in patients with CEA-positive metastatic cancer (www.clinicaltrials.gov; keywords TRICOM and radiotherapy). It has been shown that chemotherapy, like radiation therapy, can also upregulate surface molecules on tumor cells such as MHC class I and multiple TAAs [33,54–56]. Preclinical results showed that when used with a whole-tumor cell vaccine, cyclophosphamide, doxorubicin, and paclitaxel increased the immune response to the tumor and increased the delay in tumor growth over that seen with vaccine alone [57]. A phase II clinical trial combining the use of the

104


chemotherapeutic agent docetaxel with vaccine was conducted in patients with metastatic androgen-independent prostate cancer. In this trial, patients received either vaccine (an admixture of rV-PSA and rV-B7-1 along with GM-CSF) and docetaxel or vaccine alone, and PSA-specific immune responses were monitored [58]. After 3 months of therapy, the median increase in PSA-specific T cells was 3.33-fold in both arms. As seen in the trial mentioned above using radiotherapy, immune responses were detected against prostate cancer antigens not found in the vaccine. The median progression-free survival was 6.1 months in patients who received docetaxel after vaccine therapy, while a historical control reports a median progression-free survival of 3.7 months after docetaxel therapy alone (Table 7.1) [58,59]. Currently, there are several clinical trials using the combination of chemotherapy (doxorubicin, cyclophosphamide, paclitaxel, docetaxel) and vaccine, primarily in metastatic disease settings (www.clinicaltrials.gov; keywords TRICOM and chemotherapy). These trials will help optimize the combination of vaccine and chemotherapy to improve antitumor responses and hopefully extend survival in a variety of disease settings.

7.6

MODIFIED VACCINIA ANKARA

Modified vaccinia Ankara (MVA) is a replication-defective strain of vaccinia virus that was created by serially passaging vaccinia virus through chicken embryo fibroblast cells. The resulting virus is not able to replicate in mammalian cells, but has retained its ability to infect cells and synthesize viral proteins, thus remaining immunogenic [16]. These properties have made MVA an attractive vaccine vector for cancer therapy, as MVA has been shown to be safely administered to high-risk patients. In a preclinical study to determine whether MVA was as effective as vaccinia virus at inducing immune responses to tumor antigens, a recombinant MVA expressing the three costimulatory molecules of TRICOM and the tumor-associated antigen carcinoembryonic antigen (CEA; the resulting virus was designated MVACEA/TRICOM) was created [60]. Mice transgenic for CEA were primed with either MVA-CEA/TRICOM or a recombinant vaccinia virus expressing the same transgenes (rV-CEA/TRICOM). Because it had previously been shown that priming with vaccinia virus and boosting with an avipox virus elicited a greater T cell response than when avipox was given multiple times [29], mice were boosted 3 times with a recombinant fowlpox virus also expressing these transgenes (rF-CEA/TRICOM). Mice that were primed with MVA-CEA/TRICOM had a significantly greater CEAspecific CD4+ and CD8+ T cell response as compared with mice primed with rV-CEA/TRICOM. To determine antitumor efficacy, mice with established peripancreatic metastases were treated with either MVA-CEA/TRICOM or rV-CEA/TRICOM followed by three rF-CEA/TRICOM boosts, and mouse survival was observed (Fig. 7.3). There was no statistical significance in survival between mice treated with MVA-CEA/TRICOM (71%, or five of seven mice) and those treated with

MODIFIED VACCINIA ANKARA

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Figure 7.3. Therapy of 14-day established experimental metastases. CEA-Tg mice were transplanted with tumor intrasplencially on day 0 (solid arrow). On day 14 mice received the following doses: (a) MVA-CEA/TRICOM prime vaccination (M) with recombinant GM-CSF and low-dose IL-2 followed by three weekly boosts with rF-CEA/TRICOM (F) admixed with rF-GM-CSF and low-dose IL-2 (closed circles); control CEA-Tg mice received an MVA-TRICOM (no CEA) prime vaccination with cytokines followed by three weekly boosts with rF-TRICOM (no CEA) with cytokines (open circles); rV-CEA/TRICOM prime vaccination (V) with cytokines followed by three weekly boosts with rF-CEA/TRICOM (F) with cytokines (closed squares); control CEA-Tg mice received an rV-TRICOM (no CEA) prime vaccination with cytokines followed by three weekly boosts with rF-TRICOM (no CEA) with cytokines (open squares). Panels (c) and (d) show CEA-Tg mice that were prevaccinated with high-dose V-WT; 6 days later, the mice were transplanted with tumor on day 0 (solid arrow), and 14 days later (c) mice were treated as described in panel (a); (d) mice were treated as described in panel (b). Mice were monitored for survival. (Taken from Ref. 60.)

rV-CEA/TRICOM (57%, or four of seven mice). The results obtained in this study show that MVA is at least as effective as vaccinia virus at eliciting an immune response to tumor antigens and reducing tumor burden, and provides a rationale for the further study of MVA as a cancer vaccine in the clinic. Several clinical trials using MVA for cancer therapy have been conducted (Table 7.1). A recombinant MVA virus expressing the TAA 5 T4 (TroVax) has been tested in the clinic. Preclinical results were encouraging, with the number of lung metastases reduced by > 90% after treatment with TroVax as compared with treatment with MVA alone [61]. A phase I/II clinical trial was conducted in patients with metastatic colorectal cancer [61]. In this trial, patients were given TroVax once

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every 4 weeks for a total of three vaccinations. Of 17 evaluable patients, 16 demonstrated 5 T4-specific immune responses. Five patients had stable disease for 3–18 months after therapy, and all of these patients showed 5 T4-specific immune responses. There was also a positive correlation between a 5 T4 antibody response and patient survival or time to progression, and the vaccine was well tolerated [61], warranting further clinical trials with TroVax. Currently, there is a promising ongoing phase II trial with TroVax and IL-2 in renal cell carcinoma (RCC) patients, and recruitment is underway for a phase III trial in RCC patients (Oxford Biomedica Website; http://www.oxfordbiomedica.co.uk/).

7.7

CONCLUSIONS

While numerous viral vectors have been explored for the treatment of cancer, none have yet been approved by the FDA. By the time most patients are enrolled in many of the previously conducted clinical trials, they have failed other standard-of-care practices, and often present with advanced metastatic disease. More recent clinical studies have shown that poxvirus vectors can be safely combined with the current standard-of-care therapies of radiation and chemotherapy; thus, in future trials it is anticipated that these vaccines will be administered earlier in the disease process, optimizing their efficacy.

ACKNOWLEDGMENTS We would like to thank Debra Weingarten for her editorial assistance in the preparation of this manuscript.

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8 IMMUNOTHERAPEUTIC STRATEGIES AGAINST CANCER USING Listeria monocytogenes AS A VECTOR FOR TUMOR ANTIGENS Nicholas C. Souders, Thorsten Verch, and Yvonne Paterson

8.1

INTRODUCTION

Cancer is a leading cause of death worldwide, and for ages humankind has been in desperate need of a therapy that could potentially eliminate all types of cancer. Since there has been renewed interest in harnessing the immune system to combat cancer (as reviewed in the other chapters in this volume), many, including the present authors, have tried to create novel immunotherapeutics to alleviate disease. This chapter covers the use of Listeria monocytogenes as a vaccine vector for tumor antigens. We will focus on the requisite background knowledge necessary to understand certain aspects of Listeria vaccine development as a whole including a brief review of the major therapies that have used this approach thus far. We include some critical methodology and close with remarks on safety issues and future prospects for this approach against cancer. Cancer Vaccines and Tumor Immunity Edited by Rimas Orentas, James W. Hodge, and Bryon D. Johnson Copyright © 2008 John Wiley & Sons, Inc. 113

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8.2

Listeria monocytogenes AS A VECTOR FOR TUMOR ANTIGENS

LIVE RECOMBINANT VACCINE VECTORS

Recombinant vector systems based on viruses or bacteria are a very attractive tool since they induce a powerful innate immune response as well as cellular immunity. Thus passenger antigens are presented in an optimized cytokine environment to the immune system. A disadvantage of viral vectors is the generation of neutralizing antibodies against the vector backbone, which prevent further boosting steps. Intracellular bacteria can potentially address some of the shortcomings of viruses without giving up the intrinsic advantages of live vector systems. Repeated immunizations with intracellular bacteria are possible since neutralizing antibodies do not play a major role in clearing most intracellular bacterial infections [1,2]. Safety is another concern with viral vectors since adverse events cannot be readily controlled, leading in extreme cases even to death of the patient [3]. As bacteria are readily controlled by antibiotics, which can stop vector replication at any given time, their in vivo application is likely to be safer. Besides the induction of adaptive immunity, bacterial infections result in a strong innate immune response (Fig. 8.1). Conserved antigenic patterns in bacteria such as unmethylated CpG DNA, flagellin, and lipid and carbohydrate structures in the bacterial cell wall serve as danger signals to activate innate immunity, resulting in local inflammation and the influx of macrophages, neutrophils, and dendritic cells. Bacteria can be controlled by both antibodies and cytotoxic T cells depending on the species and the type of bacteria used. These mechanisms can also be used to direct the immune system against tumor antigens resulting in enhanced immunogenicity when delivered by bacterial vectors. Intracellular bacteria have attracted special interest as vaccine delivery vectors because of their ability to target antigen presenting cells (APCs) directly. While Salmonella and Mycobacteria remain in the phagosome, which they are able to modify in order to escape lysis, Listeria and Shigella avoid the antimicrobial environment of the phagolysosome by escaping into the cytosol (Fig. 8.1). This has important consequences on antigen presentation as peptides from the phagolysosome usually are presented in the context of MHC class II, whereas antigens derived from the cytosol are processed in the ER and loaded on MHC class I [4]. Listeria monocytogenes as a Vaccine Vector Listeria monocytogenes (LM) is a foodborne pathogen capable of colonizing a number of organs, which can result in death for an animal or, more rarely, a person [5,6]. Not only does the Listeria bacterium act as a natural adjuvant capable of eliciting a powerful cell-mediated immune response, but in our approach to creating attenuated Listeria immunotherapeutics, the bacterium is genetically engineered to secrete the tumor associated antigen fused to a molecular adjuvant that also enhances the overall immune response. Listeria monocytogenes infects phagocytic cells such as macrophages, neutrophils, and dendritic cells and will induce a potent CTL response downstream from the initial course of infection [7]. After


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(a) +

IL-12 Mφ or DCs

+

TNF-a

Innate Immunity

NK

IL-2 IL-1

+

Th1 IFN-γ

Neu Th0 +

IL-12

+

+

+ +

Activated Mφ or DC

IL-12

Infection

(b)

c

b

a

CD4+ T cell APC

d

g e

Adaptive Immunity

f

h

CD8+ T cell

Figure 8.1. Infection with Listeria monocytogenes results in innate and adaptive immunity. Panel (a) shows bacteria taken up by phagocytic cells including macrophages (Mφ), dendritic cells (DCs), and and neutrophils (Neu). Cell wall and other components activate macrophages to produce IL-12 and TNF-α to activate NK cells, and IL-1 to activate neutrophils. Activated NK cells produce IFN-γ, which acts on macrophages to upregulate antigen-processing machinery and increase the further production of IL-12. This burst of inflammatory innate immunity drives the maturation of Th0 cells to Th1 cells. Panel (b) shows that phagocytosed L. monocytogenes that are unable to lyse the phagosomal–lysosomal membrane are degraded (a) and their proteins are broken down into peptides (b) that can be loaded onto MHC class II molecules (c) and presented to CD4+ T cells. Alternatively, if the bacteria succeed in lysing the phagosomal membrane (d), they and their secreted virulence factors are released into the cytoplasm (e). These secreted proteins are cleaved by the proteosome (f), and the resulting peptides are transported into the endoplasmic reticulum, where they are loaded onto MHC class I molecules (g). The class I MHC molecules are sent to the cell membrane (h) where they present the peptides to CD8+ T cells.

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phagocytosis of Listeria, the bacteria can escape from the phagolysome into the cytosol of the APC by producing the hemolytic protein, LLO, which is degraded in the cytosol by the proteosome to prevent host cell lysis [8]. In the cytosol, Listeria expresses the actin-polymerizing protein, ActA, which allows it to spread from cell to cell [9]. These virulence factors, LLO, ActA, and others, are under the control of a pluripotential transcription factor, PrfA [10,11]. Once Listeria escapes the phagolysome, it can induce the production of IL-12, IL-6, and TNFα and enhance DC maturation [12]. In addition to inducing the production of nitric oxide (NO) by macrophages, Listeria can also induce a rare DC subset to produce NO and TNFα [13]. Although antibody responses to Listeria can be measured in some model systems, cellular immunity is the key to clearing infections. We first described the potential of L. monocytogenes as a vaccine vector in the early 1990s [14,15], but it has attracted the interest of an increasing number of researchers as a potentially ideal vaccine vector more recently [1,16–26] since its unique lifecycle and low pathogenicity set it apart from Salmonella and Shigella. Some of the basic principles of the methodology described for Listeria later in this chapter can be easily adapted to other bacterial species with some modifications that can be obtained from the literature. The interested reader may also consider several review articles on bacterial vector systems as a guide through the literature [27–32]. For many cancers, immunotherapeutic approaches are focused on inducing a CTL response that can kill tumor cells that consistently express a tumor associated antigen required for maintenance of the malignant phenotype [33]. The generation of tumor-infiltrating lymphocytes, especially tumor-specific CD8+ effector T cells, can be used as a surrogate marker for vaccine efficacy [24,34,35]. As we have shown previously, Listeria monocytogenes can be used to eliminate palpable, vascularized tumors in several mouse models because of the vaccine’s ability to generate tumor-infiltrating lymphocytes [20,24,25,35]. A drawback to many cancer immunotherapeutics is their lack of applicability to human tumor antigens with homology to self-antigens due to immune tolerance mechanisms, which limit patients’ immune responses to their cancers [36,37]. Tolerance allows for the growth of tumors, which have gained the ability to divide very rapidly while being resistant to death, because they are ignored by the immune system. In order for a cancer immunotherapeutic to be effective, it will have to overcome tolerance mechanisms and activate self-reactive T cells to target the tumor. The ability of Listeria-based vaccines to break tolerance has not been extensively examined. Listeria that secrete the self-melanoma antigen, Trp2, have been tested in transplantable mouse melanoma models both alone [38] and in conjunction with the TLR7 agonist imiquimod [39]. Although, the vector could protect mice against melanoma challenge [38,39], the level of tolerance to this antigen could not be examined because there is no mouse syngeneic to the tumor that lacks this antigen. Attempts to break tolerance to a self-antigen using Listeria targeted the endogenous gp70 retroviral gene in the CT26 mouse tumor model; however, the CTL epitope that this strain secreted did not have complete homology with the target gp70 sequence and therefore cannot stringently be considered a self-antigen [16].


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Studying novel cancer immunotherapies in a transplantable mouse model is a first approach to evaluating the effectiveness of a therapy. However, if wild-type mice are used as recipients, there may be little or no tolerance unless a self-antigen is expressed by the tumor. A better test of cancer immunotherapies may be to test them in tolerance models, such as transgenic mouse models for cancer [40,41]. Effective treatments in animals that are tolerant to the tumor antigen will theoretically have a greater chance of being effective in human clinical trials. In the paragraphs that follow we will describe our studies using the human tumor antigens, HPV-16 E7 and HER2/neu and Listeria vectors, which at least partially break tolerance. Listeria as a Vaccine for Cervical Cancer Human papillomaviruses 16 and 18 are the primary causative agent of anogenital cancer and have been strongly associated with certain forms of head and neck cancer. In fact, over 50% of both cervical cancer and squamous cell carcinoma of the tonsil are associated with HPV-16 [42–44]. Cervical cancer is the second most common female malignancy-inducing disease in over 400,000 women per year worldwide, and is a leading cause of death when untreated. Of all oral cancers unrelated to tobacco or alcohol usage, 20% are associated with HPV infections [43,44]. Two genes, E6 and E7, are responsible for the induction of cancer by downregulating the tumor repressor genes Rb and p53 and immortalizing the epithelium of the cervix. E6 degrades p53, and E7 binds to and degrades Rb [45–47]. Evidence has accumulated in humans that immune responses can be directed against HPV16 [48,49]. One of the immortalizing proteins in HPV-induced cancers is the Rb-binding protein, E7, which has been well characterized in a number of studies not limited to the ones mentioned below [17,34,50–54]. We created two Listeria-based vaccines, LM-LLO-E7 and LM-ActA-E7, as potential vaccines for HPV-16 induced cancers [17,24]. A truncated listeriolysin O (LLO) is fused to E7 in LM-LLO-E7, whereas in LM-ActA-E7, the E7 gene is fused to a truncated version of the ActA protein. We routinely fuse cancer antigens to listerial virulence factors to improve antigen presentation in vivo. Both the truncated LLO and ActA have PEST (proline, glutamic acid, serine, threonine)-like domains within their amino acid sequences, which have been postulated to result in the rapid degradation of proteins in the host cell cytosol [55,56]. A putative PEST-like domain at the N -terminus of LLO (amino acids: NKENS ISSMA PPASP PASPK TPIEK KHADE ID) is suspected to degrade this toxic protein by directing it to the proteosome [8], although there has been more recent evidence of the involvement of other mechanisms [57]. The fusion of antigens to LLO or the PEST-like sequence of LLO or ActA seems to enhance the immunogenicity of the fused antigen [17,23,24]. In order to avoid overexpression of toxic LLO, we truncate the protein at AA 441, thereby deleting the hemolytic domain. LM-LLO-E7 and LM-ActA-E7 have been shown to induce CD8+ T cells that can home to, penetrate, and kill the solid tumor formed by TC-1 cells in a normal C57BL/6 mouse [17,24,35]. The TC-1 cell line is a C57BL/6 immortalized primary

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lung epithelial cell line that was transformed with E6, E7, and c-Ha-ras oncogenes [52]. TC-1 cells are poorly immunogenic in vivo and form solid tumors when injected subcutaneously into syngeneic host mice, which cannot control the growth of the tumor without some type of intervention [52]. We found that E7 alone delivered by Listeria, LM-E7, induced formation of CD4+ CD25+ regulatory T cells in the tumor, whereas LM-LLO-E7 prevented the formation of CD4+ CD25+ regulatory T cells [35]. Both the Lm-LLO-E7 and Lm-ActA-E7 vectors could effectively eradicate established TC1 tumors of a measurable size in the wild-type C57/BL6 mouse. We created a mouse transgenic for HPV-16 E6 and E7 under the thyroglobulin promoter in order to test the ability of the vaccines to break tolerance. The E6 and E7 genes are expressed in the thyroid and in mouse thymic epithelial cells. We showed that the mouse appeared to demonstrate central tolerance to E7, since the T cells induced in response to these vaccines were of lower avidity than those induced in wild-type mice. Nevertheless, the vaccines were able to eradicate established TC-1 tumors in the E6/E7 transgenic mouse, albeit at a lower frequency than in the wild-type mouse [58].

Listeria Expressing HER2/neu as a Vaccine for Breast Cancer HER2/neu is a member of the epidermal growth factor receptor family of tyrosine kinases and has three distinct domains: an extracellular domain, a transmembrane domain, and an intracellular domain [59,60]. The formation of breast tumors can be enhanced by HER2/neu, making it a good target for immunotherapy [61]. HER2/neu-expressing mouse tumors have been derived from the FVB mouse transgenic for the HER2/neu gene under the control of the MMTV promoter [62]. This transgenic mouse exhibits profound tolerance to HER2/neu since the transgene is expressed in hematopoietic cells and the thymus [63]. We created five Listeria vaccines, each of which expressed and secreted overlapping fragment of HER2/neu [25,64,65]. Both the extracellular and intracellular domains were able to be included in this vaccine, and all portions of HER2/neu were fused to our molecular adjuvant, the truncated, nonhemolytic LLO. The highly hydrophobic transmembrane domain cound not be secreted by Listeria [25]. All five vaccines spanning the length of the extracellular and intracellular portions of HER2/neu were equally able to induce the regression of an implantable tumor cell line that expresses HER2/neu in the wild-type FVB mouse [25]. In previous studies in the FVB mouse, only one CTL epitope had been described [62]. However, we were able to discover a number of novel epitopes using our five Listeria vaccines [64,65]. In addition we have shown that each of these vaccines can control the growth of an HER2/neu-expressing transplantable tumor in the FVB mouse transgenic for the HER2/neu gene under the control of the MMTV promoter [65], providing further evidence that Listeria can overcome tolerance and expand low-avidity T cells for the treatment of tumors expressing self-antigens.


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Listeria as a DNA Cancer Vaccine More recently, a novel bactofection vector using Listeria as a DNA-based cancer was generated in which bacteria deliver a plasmid encoding a tumor antigen under the control of a mammalian promoter in an attempt to induce an antitumor immune response [66]. Bactofection, or bacteria-mediated intradomain gene transfer, has been proposed as a potential means to develop novel therapeutics to treat a variety of human diseases including cancer [67,68]. There are several potential advantages to using Listeria as a bactofection agent to deliver a DNA vaccine as it could circumvent some limitations of the Listeria-based protein secreted vaccines. Empirically, we have found that Listeria cannot secrete highly hydrophobic proteins, presumably because of their difficulties in translocating across the membrane or cell wall. Additionally, we have observed that there is an upper limit on the molecular weight of the eukaryotic protein to be secreted of about 50 kDa (YP, unpublished observations). The use of a Listeria-based DNA vaccine could avoid the complication of soliciting a prokaryote to secrete a eukaryotic protein by diverting the expression of the protein of interest to a eukaryotic host cell. Utilizing a plasmid release mechanism involving the suicide of the carrier bacteria [69,70], we were able to engineer Listeria monocytogenes to induce antitumor immunity to the cervical cancer oncoprotein, E7 [66]. In the TC-1 model, we were able to slow tumor growth and induce a cytotoxic CD8+ T cell response against the RAHYNIVTF epitope for E7. However, although this delivery system was more effective than administering plasmid alone, it was not as effective as LM-LLOE7, which was engineered to deliver the E7 protein in impacting on established tumor growth [66]. Constructing Listeria monocytogenes Vectors Most vector strains have been based on L. monocytogenes strain 10403S, a natural streptomycin-resistant strain derived from 10403 [71]. Its sequence is very close to the EGD strain, which can be used as a basis for primer and cloning design. All genetic engineering steps need to be performed in Escherichia coli since the transformation efficiency of Listeria is very low and requires relatively large amounts of purified plasmid DNA (approximately 1 μg). Several shuttle plasmids have been published, of which we suggest the use of two different types (Fig. 8.2), depending on whether integration of the recombinant gene of interest into the Listeria genome is desired. This can be achieved by using pKSV7, which contains a temperature-sensitive origin of replication [72]. This plasmid was modified by introducing approximately 500 bp (base pairs) of Listeria open reading frame (ORF) Y and ORF Z interrupted by a multiple cloning site for the recombinant gene in order to allow for integration specifically into the ORF XYZ genome region (Fig. 2a) [73]. The second method for transforming Listeria, which is preferred in our laboratory, is to insert a required gene for in vivo bacterial growth on the plasmid to

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(Plasmid) X

X

LLO-E7

LLO-E7

Y

Y

Z

(Genome) (a) LLO-E7

prfA

R(−)

R(+)

CAT(−)

CAT(−)

(b)

Figure 8.2. Expression of foreign genes in Listeria (here as an example: HPV E7 genetically fused to LLO (LLO-E7)): (a) Genome-based gene expression: schematic map of genomic integration. The LLO-E7 expression cassette on a plasmid is flanked by 300–1000 bp of genome sequences from the integration site. The gene of interest will then be inserted into the genome by homologous recombination at these sites. (b) Plasmid-based gene expression: schematic map of a E. coli–Listeria shuttle plasmid. The LLO-E7 fusion gene is driven by the LLO promoter. This plasmid is designed to complement a prfA-deficient mutant Listeria strain. The prfA gene could be replaced with alternative in vivo selection genes as we discuss in the text. Also included in this specific example are CAT( − ), CAT( + )—chloramphenicol transferase genes for in vitro selection in gram-negative and gram-positive bacteria; R( − ), R( + )—replication genes for gram-negative and gram-positive bacteria.

avoid plasmid loss on immunization. We have used the pluripotential transcription factor PrfA to complement a mutant strain in which the prfA virulence gene has been inactivated [15,17,25]. Plasmid-based gene expression can be achieved using shuttle plasmid pAM401 [74], which contains replication and antibiotic resistance selection genes for both E. coli and Listeria (Fig. 8.2b). Recombinant gene expression was made possible by introducing the Listeria hly promoter and the prfA virulence gene resulting in pDP2028 [15]. We have modified this approach by removing the antibiotic-resistant genes and substituting an alanine racemase gene for prfA [75]. Using this plasmid to complement a Listeria strain, LmDD , which lacks genes that control the synthesis of the listerial cell wall [76], allows for selection of transformed Listeria both in vitro and in vivo. The p60-dal in trans complementation system has been developed to allow for bacterial plasmid retention in an antibiotic-free manner [75]. Since the shuttle plasmids are relatively


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large, all initial polymerase chain reaction (PCR)-cloning steps are performed in TA-cloning vector pCR2.1 (Invitrogen) prior to moving the recombinant genes into pKSV7 or pAM401. The expression levels of the recombinant antigen that can be obtained in bacteria containing only a single gene copy are lower than those that can be obtained from a multicopy plasmid. While episomal gene expression could be advantageous due to the presence of multiple gene copies, it requires careful consideration of plasmid stability in vitro as well as in vivo. Since direct cloning into Listeria has been tedious in our and others’ experience, all genetic manipulations and amplification of plasmid material should initially be performed in E. coli prior to electroporation of the final plasmid into Listeria. For laboratory research, convenient shuttle plasmids that contain antibiotic resistance genes for in vitro selection of both bacteria have long been available [74]. As these plasmids are rapidly lost in vivo, where no antibiotic pressure is present, our group has used complementation systems for in vivo stabilization. The addition of the prfA gene into the plasmid in combination with a Listeria strain lacking this gene helped to dramatically improve plasmid stability in vivo [15]. Similar complementation systems using other genes have been described by our group as well as others [70,75]. Genetic manipulations and extended in vitro culture leads to the downregulation of general virulence of Listeria. Efficacy can be enhanced by passaging vaccine strains in mice [71,77]. Besides upregulation of bacterial virulence, this results in stronger immune responses against the bacteria and the recombinant antigen [78]. The passaging step needs to be performed only twice in order to establish a cell bank that can be used subsequently to produce fresh immunization material as needed. As most vaccine strains are attenuated either deliberately for safety compliance or because of the recombinant antigen, a larger amount of bacteria can be injected into mice than applicable for wild-type Listeria. We routinely start with 107 –108 colony-forming units (CFUs) injected intraperitoneally (IP). On day 3, the spleen is isolated, then homogenized in 5–10 mL of PBS, and 50–200 μL are plated in order to recover single colonies. Four colonies are pooled and expanded in liquid culture. The bacteria are frozen in 20% glycerol, and a dilution series is prepared for CFU counts ranging from 1 : 101 to 1 : 1010 (plate 5–10 μL). A second passage is then performed similarly using the injection stock prepared from previously passaged bacteria. The greatest benefits to bacterial virulence and immunogenicity are observed after two passages in vivo [78]. We routinely prepare vaccine stocks after two passages and determine the virulence of this final stock. Because, in contrast to viruses, intracellular bacteria such as Listeria seldom evoke neutralizing antibodies in the host, multiple vaccination boosts are possible. We have tried up to four immunizations of 0.1 × LD50 in mice resulting in increasing vaccine efficacy [1]. Because of the buildup of some immunity against the bacterial vector, higher booster doses can be chosen than those used for primary immunization without increasing the toxicity, specifically, 0.2 or 0.3 × LD50 . Unless lack of efficacy demands a more extensive immunization protocol, we routinely immunize twice in 1-week intervals with 108 CFU (about 0.1 × LD50 for most strains).

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The route of immunization depends on the type of immune response that needs to be achieved and on practical considerations. As most tumors do not require a mucosal response, we usually inject the vaccine IP or IV, both of which are efficient in the E7 tumor system (unpublished data). Either route of injection has resulted in similar antitumor immunity. Although subcutaneous (SC) immunization may appear to be a logical, convenient, and safe route of immunization for a SC tumor, we found decreased efficacy using this route in tumor regression studies. The oral route is difficult to test since Listeria do not readily cross the gut barrier in mice. Guinea pigs are a more suitable study subject [79], but cancer models have not been well established in these animals. Using a less attenuated vaccine strain (LD50 : 1 × 107 ) than usual (LD50 : 1 × 109 ), we were able to immunize orally against challenge with a tumor cell line expressing influenza nucleoprotein [21], and similar results were reported for the E7 model system [19]. Additional attempts with more attenuated strains, however, have not yet been successful in our hands.

8.3

SAFETY CONCERNS

Biologics and live vectors in particular are subject to close scrutiny for patient and environmental safety. Although live attenuated bacteria have been used as vaccines against the pathogenic version of the bacterial strain for several decades, their use as antigen vectors has entered the field only recently (as of 2007) and with limited applications. Therefore, bacterial vectors are often regulated together with DNA vaccines or viral vectors for which extensive guidelines are available. As with other vaccines, oral or subcutaneous application is preferable to IV injection from a regulatory standpoint. The Listeria vector itself should not display pathogenicity. It has to be sufficiently attenuated without the recombinant antigen present. Attenuation caused by the antigen is not considered suitable for safety evaluation. Usually a difference of at least 2–3 log units between the efficacious dose and a pathogenic dose is preferred. The attenuation has to remain stable in a clinical setting to avoid reversion to the wild type during the time of therapy. The attenuation as well as the general geno/phenotype should be well characterized, which was seldom the case for laboratory strains that were transferred between different groups over several years. For a new group embarking on using Listeria as a vaccine vector, it is recommended to start out with a strain obtained from a type culture collection such as ATCC. Any live vector could potentially cause serious adverse reactions in patients. Most of these can be avoided by using stably attenuated strains and careful examination of patients for immune deficiencies. A number of rationally attenuated mutants of Listeria have been developed as vaccine strains, which have been previously reviewed [30]. In general, live replicating vectors are thought to result in more profound immunity [14,80]. However, the notion of using bacteria that are unable to replicate in vivo has recently been revisited. One group has developed a new type of killed Listeria vaccine with enhanced safety in which the bacteria are killed by psoralens and UV light, but the Listeria remains metabolically active and

CLOSING REMARKS

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capable of protein production for some time after exposure to the psoralens [81]. Listeria killed by γ irradiation has been shown to activate DCs and prime CD8+ T cells by cross-presentation and protect against live Listeria challenge [82]. In our experience, listeriolysin O–negative strains have not been efficient host strains for passenger antigens [15], presumably because of the inability of the bacterium to enter the host cell cytosol and access MHC class I Ag processing machinery. More recently, however, a LLO negative strain of Listeria that expresses a CD8+ T cell epitope derived from lymphocytic choriomeningitis virus was found to generate memory, epitope-specific CD8+ T cells. Mice primed with this strain resisted challenge with virulent Listeria and lymphocytic choriomeningitis virus [83]. However, whether any strains that cannot replicate in vivo are as competent for the delivery of tumor antigens as attenuated replicating strains has not yet been examined and requires further investigation. In contrast to virus vectors, bacteria offer an additional possibility for control because of their sensitivity to antibiotics. As the injected bacterial strain is known and characterized, potentially adverse reactions could be immediately addressed by the application of a suitable antibiotic before the bacterial infection is beyond control. For example, most of the safety concerns raised for Listeria-based vectors such as the possibility of meningitis can easily be addressed this way. In general, clinical protocols for the application of bacterial vectors should contain an antibiotic dose at the assumed end of the vector infection for safety reasons. Sometimes concerns are also raised for the safety of healthcare workers and patients’ families because of a potential spread of the vaccine vectors from person to person. This may, for example, represent a concern in Shigella or Salmonella vectors based on typhoid strains when not sufficiently attenuated but should not pose a risk for Listeria since person-to-person transmission of even wild-type strains does not appear to occur even in clinically confirmed cases of Listeriosis (Bennet Lorber, personal communication). Besides the safety concerns for caretakers, potential environmental spread of the vector should also not represent a hazard. Therefore, the FDA strongly discourages the presence of any antibiotic-resistance genes in the final vector. The recombinant antigen needs to be maintained stably either on a plasmid or integrated into the bacterial chromosome without antibiotic selection. As these concerns will be raised whenever a bacterial vector is moved into clinical applications, vector design should address them early in the preclinical phase.

8.4

CLOSING REMARKS

Listeria-based cancer vaccines have proved to be relatively easy to create. We have tested numerous strategies over the years and found that Listeria-based cancer vaccines, due to the unique lifecycle of Listeria and the type of immune response that it elicits, are effective in murine models in eradicating solid, well-vascularized tumors. We are thus optimistic about their potential for treating human disease. The cervical cancer vaccine, LM-LLOE7, is currently undergoing a Phase I clinical trial,

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and we expect the vectors to demonstrate no toxicity. Additionally, the finding that Listeria-based HER2/neu vaccines can elicit a therapeutic immune response even in mice for which this is a self-antigen [25,64,65] encourages us to begin to translate our HER2/neu vaccines from the mouse and into the human.

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9 COUPLING INNATE AND ADAPTIVE IMMUNITY WITH YEAST-BASED CANCER IMMUNOTHERAPY Sibyl Munson, Joanne Parker, Thomas H. King, Yingnian Lu, Victoria Kelley, Zhimin Guo, Virginia Borges, and Alex Franzusoff

9.1

INTRODUCTION

Many different strategies for the generation of immune responses using cancer vaccines or cancer immunotherapy are being employed for the treatment and/or prevention of human tumors. The two major elements of cancer immunotherapy are the selection of the antigens used to focus the specificity of the immune response and the approach or vehicle employed to stimulate the immune system against these antigens to eliminate target antigen-bearing tumor cells. Recombinant nonpathogenic brewer’s yeast, Saccharomyces cerevisiae, which we call Tarmogens™ (acronym for targeted molecular immunogens), are attractive vectors for cancer immunotherapy for the following reasons: (1) yeast trigger both innate and adaptive immune responses and therefore do not require additional adjuvants; (2) yeast deliver polypeptide antigens that are effectively processed into a Cancer Vaccines and Tumor Immunity Edited by Rimas Orentas, James W. Hodge, and Bryon D. Johnson Copyright © 2008 John Wiley & Sons, Inc. 131

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full complement of appropriate-sized peptides competent for presentation by MHC class I and class II pathways, irrespective of HLA type; (3) yeast elicit potent T cell immune responses against tumor cells expressing target antigens; (4) yeast are not neutralized by the host immune system, even on repeated administration; (5) yeast-based immunotherapy is not a custom-manufactured, patient-specific vaccine, yet multiple tumor antigens may be targeted, so that treatment may be customized with a yeast “cocktail” dependent on a patient’s tumor genotype profile; and (6) recombinant yeast are simple to manufacture. This chapter discusses methods for engineering yeast to express tumor antigens and the unique properties of recombinant yeast in the activation of innate and adaptive immune responses. The broad applicability of the yeast-based immunotherapy to elicit protective T cell immune responses has been demonstrated in preclinical studies with numerous foreign, mutated, and overexpressed antigens. For instance, the therapeutic administration of yeast expressing mutated Ras protein triggered the complete ablation of carcinogen-induced mutant Ras-bearing tumors in mice. The safety and immunogenicity of yeast Tarmogens in cancer patients are also discussed. 9.2 YEAST-BASED IMMUNOTHERAPY: MOLECULAR MECHANISMS OF ACTION Yeast delivering tumor antigens activate innate immunity as well as the adaptive antigen-specific immune responses. The dual contributions to immune activation arise from the inherent “adjuvant-like” properties of yeast combined with direct delivery of the expressed tumor antigen for processing and presentation to the immune system. Yeast components activate the innate immune system by transmitting the “danger signals” of microbial infection through distinct surface receptors on APCs that recognize particle-associated molecular patterns (PAMPs) by pattern recognition receptors. These pattern recognition receptors include toll-like receptors (TLR1,2,4,6) and phagocytic receptors, such as dectin, mannose, and glucan receptors [1–3]. The inherent recognition of yeast component patterns as a “foreign” infection is not restricted to S. cerevisiae, but is shared with other fungi [1,4,5]. Saccharomyces cerevisiae yeast cells are avidly phagocytosed in vitro by murine bone-marrow-derived dendritic cells DCs, human myeloid DCs and human plasmacytoid DCs, macrophages, and neutrophils, as well as human EBV-immortalized B cells [6,7] (See also Munson S, Parker J, Franzusoff A, unpublished observations). The uptake of multiple yeast per phagocytic cell has been observed within a few (0.5–6) hours after exposure, as determined by flow cytometry and fluorescence microscopy. The uptake of yeast by DCs triggers DC activation, maturation (including upregulation of cell surface receptors and secretion of various cytokines), and the presentation of peptides from yeast-expressed tumor antigens on MHC-I and MHC-II receptors, leading to an antigen-specific response by CD8 and CD4 T cells in the immunized hosts [8]. The elements of DC activation and antigen processing triggered by yeast exposure are elaborated in the following sections.

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DC Activation and Maturation The interaction of innate signaling receptors TLR1,2,4,6, plus phagocytic receptors on the surface of APCs, appears to be a combinatorial process that enhances and modulates the innate responses that would otherwise be achieved by binding individual innate receptors [9,10]. The consequences of TLR recognition of yeast molecular patterns by DCs results in the expression of genetic responses that are distinct from the profile elicited in response to bacterial components, viral components, or individual TLR-ligand agonists [11]. TLR2 interacts with zymosan (preparations of S. cerevisiae yeast cell walls) in conjunction with TLR1 and TLR6 [12,13]. TLR4 interacts with mannan on the surface of Candida yeast, and presumably on the surface mannan of S. cerevisiae [12,14]. Other receptors apparently involved in S. cerevisiae recognition and response are the β-glucan receptor (dectin-1 in the mouse), mannose receptor, complement receptor 3 (CR3), and the scavenger receptor CD36 [14–19]. The consequences of these multiple receptor interactions with yeast are the initiation of endocytic and phagocytic mechanisms in DCs, the activation of phagolysosomes that process the internalized yeast cells, degradation of the yeast in the endosomes and phagolysosomes, presentation of exogenous yeast-associated peptides into the MHC-II receptor pathway (within specialized endosomes), and activation of the proteasome pathway for cross-presentation of yeast-associated peptides into the MHC-I receptor pathway. As shown in Figure 9.1, the incubation of yeast also triggers upregulation of cell surface molecules, such as CD80 (B7.1) and CD86 (B7.2) and CD83 [6,7] (Borges V, unpublished data). Stimulation of DCs by yeast also induces secretion of a proinflammatory panel of Th1-type cytokines, such as IL-6, IL-12, and TNFα [6,7,20,21]. Cross-Presentation of Exogenous Yeast-Associated Antigens into the MHC-I Pathway The classical view of MHC-restricted antigen presentation is that peptides for stimulating CD8 T cell responses via the MHC-I pathway must be derived from polypeptides endogenously expressed by the APCs. Further, this model holds that exogenous antigens are delivered exclusively by endocytosis into the MHC-II presentation pathway. More recent work has provoked revision of the classical view of MHC-I antigen presentation, since it was discovered that certain types of exogenous delivery methods were effective at loading peptides into the MHC-I pathway, and thereby stimulating activation of CD8 T cell immune responses, in a process known as cross-priming or cross-presentation [20–26]. Yeast are capable of inducing cross-priming, as shown by a variety of in vitro and in vivo studies [6,27]. In vitro, recombinant yeast expressing a model antigen, namely, ovalbumin, when incubated with DCs, were able to activate OVA-specific MHC-I restricted T cell responses. Immature murine DCs were incubated with either soluble ovalbumin alone, yeast lacking foreign antigens (YVEC),

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or yeast expressing chicken ovalbumin (OVAX). The pulsed DCs were then incubated with naive OVA-specific CD8+ T cells (obtained from OT-1 T cell transgenic mice). As expected, DCs pulsed with soluble ovalbumin or YVEC were unable to activate ovalbumin peptide-specific CD8+ T cells. In contrast, DCs incubated with OVAX activated the ovalbumin-specific CD8+ T cells and the amount of proliferation was improved in a yeast dose-dependent manner by increasing the yeast to DC ratio during pulsing [6]. Surprisingly, DCs pulsed with soluble ovalbumin mixed with YVEC also stimulated CD8+ T cell proliferation [6]. However, 40,000-fold more protein is required when provided as soluble protein plus empty yeast (YVEC), compared to the amount of ovalbumin that efficiently triggers antigen-specific CD8+ T cell responses when delivered inside the yeast. This result indicates that innate receptor agonists such as yeast are several orders of magnitude more effective at eliciting adaptive immune responses when the innate receptor agonists are physically associated or linked with the tumor antigens. Tumor challenge models in immune-competent or CD8-knockout mice, or inactivating CD8 T cells with specific anti-CD8 antibodies are in vivo examples demonstrating that immunization with recombinant yeast primes tumor-targeted CD8 T cell responses [6] (Duke R, unpublished observations). Immune-competent mice immunized with OVAX were protected against challenge

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with ovalbumin-expressing EL-4 tumor cells, but not to challenge with EL-4 cells that did not express ovalbumin (6) (see also Duke R, unpublished observations). The protection mediated by immunization with OVAX yeast was abolished in mice deficient in CD8+ T cells (6). Thus, while not all immune responses that protect animals from tumor challenge or spontaneous carcinogen-induced tumors are CD8 T cell-restricted, this study and others have demonstrated that immunization with yeast are potent inducers of CD8 T cell responses. Furthermore, the incubation of immature DCs with yeast expressing ovalbumin, but not YVEC yeast or mock treatment, was sufficient for activating tumor protective immune responses when the OVAX-pulsed DCs were introduced back into mice challenged with ovalbumin-expressing tumors [6]. These results suggest that yeast activation of APCs and antigen delivery by yeast to APCs is necessary and sufficient to explain the mechanism of yeast-based immunotherapy in immunized hosts.

9.3 MUTATED Ras AND THERAPEUTIC ANIMAL TUMOR MODELS FOR YEAST-BASED IMMUNOTHERAPY The selection of tumor antigens for targeted immunotherapy is typically based on (1) mutated proteins that arise during tumorigenesis [e.g., Ras, adenomatous polyposis coli (APC)], (2) overexpression of self-proteins specific to tumor cells [e.g., epidermal growth factor receptor (EGFR)], (3) differentiation, neo- and self-antigens whose expression is activated or restricted to tumor cells [e.g., melanoma antigens, mesothelin, carcinoembryonic antigen (CEA)], or (4) proteins expressed in the stroma that support tumor survival and proliferation (e.g., PSMA). However, given the multiple mechanisms by which tumor cells alter protein expression or trigger escape mutations under selective drug or immune pressure, the optimal antigens for initial therapeutic targets should be those that are critical for tumorigenesis or metastasis. Some of these targets have been identified as “antigens of addiction” because of their essential role in the cancer phenotype. Mutated Ras protein was selected as an ideal target antigen for yeast-based immunotherapy because of its essential role as an engine driving tumorigenesis in multiple types of human cancer. The three Ras protein members, K-, H- and N-Ras, are GTPase-switch proteins important for cell proliferation acting downstream of receptors that respond to external stimuli for cell division, such as EGFR. Single amino acid mutations in at least two domains in Ras (amino acids 12 or 13, and residues 59 or 61) are commonly associated with cancer as these mutations cause constitutive, signal-independent cell growth and tumorigenesis [28]. Mutations in K-Ras are present in 90% of human pancreatic adenocarcinomas, 35–40% of colorectal tumors, and 30–35% of non-small-cell lung cancer adenocarcinomas [29,30]. The mutated Ras proteins commonly found in cancer were expressed as tumor antigens in yeast (the GI-4000 series of yeast strains). A carcinogen-induced lung tumor model was used to test the effect of the GI-4000 yeast (31). In this system, one intraperitoneal injection of urethane in A/J mice spontaneously triggers 25–50

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individual lung tumors per mouse, each driven by single amino acid mutations in the Ras oncoprotein. Most of the individual tumors are driven by K-Ras mutated at codon 61 (i.e., Q61R or Q61L). The genotype of some urethane-induced tumors harbor mutations at K-Ras codon 12 (i.e., G12V or G12D), while a minority of the 25–50 tumors do not appear to express mutated Ras proteins [32,33] (see also Lu Y, Franzusoff A, unpublished observations). Mice display pulmonary alveolar hyperplasia by 2 weeks post-urethane exposure, adenomas by 5 weeks, and adenocarcinomas by 14 weeks post-injection. The animals expire within 12 months from tumor-associated respiratory distress. Yeast strains were engineered to express the mouse K-Ras protein with the Q61R mutation [strain GI-4000 (Q61R)], or the K-Ras protein with the Q61L mutation [strain GI-4000 (Q61L)]. After culturing the yeast and inducing expression of the mutated K-Ras proteins, the yeast were harvested, heat-killed, washed, and resuspended as intact cells in PBS. Dosing of animals is based on the number of yeast administered (1 YU = 107 yeast cells) with a known quantity of mutated Ras protein expressed per YU yeast at the time of administration. The therapeutic efficacy of yeast-based immunotherapy in the urethane-induced lung tumor model was examined by initiating dosing 2–5 weeks post-urethane exposure, a time at which 25–50 tumors per mouse are already present as hyperplasias or adenomas. Mice were administered 5 YU yeast per injection site. Animals received total 6 doses (i.e., every other week), 10 total doses (i.e., each week), or 6 injections at each of two sites every other week (i.e., 12 total doses) of buffered saline (PBS), control yeast that harbored no heterologous protein (YVEC/GI-1001) or yeast harboring the mutated Ras proteins [e.g., GI-4000 (Q61R) yeast or GI-4000 (Q61L) yeast]. At 14 weeks post-urethane exposure, where untreated tumors would be macroscopically visible, the tumors were excised, counted, and measured with a caliper to calculate the volume of each individual tumor, and the average tumor burden per mouse was determined. Compared with the saline-treated group, the tumor burden in mice that received 6 doses of yeast vaccine GI-4000 (Q61R) led to an average reduction in total tumor volume per mouse of 39%, mice that received 10 doses showed an average reduction in tumor burden of 55%, and mice that received 12 doses exhibited 52% reduction in tumor burden (Fig. 9.2) [33]. To assess the impact of targeted yeast-based immunotherapy, the K-ras sequences in the residual tumors were genotyped. The tumor-sequencing results revealed that while 56% tumors from saline or mock-yeast-treated mice harbored K-Ras Q61R mutations, the administration of increased numbers of doses of GI-4000 (Q61R) yeast led to the reduction in number of Q61R-Ras-bearing tumors to the point where 12 doses resulted in complete ablation of all tumors driven by Q61R-mutated K-Ras (Fig. 9.2). Conversely, when mice were administered GI-4000 (Q61L) yeast, the tumors bearing Q61L-mutated K-Ras were targeted for destruction by the yeast-mediated immune response [33]. Interestingly, when the two different yeast were injected in urethane-treated mice at different sites, or mixed

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Figure 9.2. Mutation-specific tumor ablation observed when yeast vaccine administered contains the Q61R or Q61L mutation. A/J mice received a single injection of urethane, then 2 weeks later received either (1) 6 doses every 2 weeks of GI-4000(Q61R) or GI-4000(Q61L), (2) 10 doses of GI-4000(Q61R) weekly, (3) 12 doses of GI-4000(Q61R) every 2 weeks, (4) mock treatment of saline, or (5) 6 doses of GI-1001(YVEC, empty yeast vector) every 2 weeks. Then 14 weeks post-urethane exposure, tumors were excised, counted, and measured, and the total tumor burden was calculated. In total, 10–30 single tumors/group were used to prepare DNA and the ras sequence determined for each. Dark bars designate the total tumor burden (left axis), whereas light bars designate the percentage of tumors sequenced that bear Q61R or Q61L mutations (right axis). The light bars left of the line represent % tumors bearing Q61R-mutated Ras, whereas light bars right of the line represent % tumors bearing Q61L mutated Ras. (Adapted from [33] and Lu Y, Franzusoff A, unpublished.)

in the syringe for injection at a single site, improved protection compared to individual yeast administration was observed, and some animals had eliminated all of the tumors, supporting the premise of combinatorial treatment by cocktails of yeast targeting different antigens (Lu Y, Fiolkoski V, Bellgrau D, Franzusoff A, unpublished observations). Taken together, these results showed that therapeutic immunization with yeast bearing mutated Ras proteins was able to activate the immune system to eliminate all tumor cells driven by a single amino acid Ras mutation with yeast-based targeted immunotherapy. Tumor-targeted immune responses were enhanced with repeated yeast immunizations, even when administered weekly. Furthermore, although humans typically harbor cancers arising from a single tumor source, these results underscore the importance of genotyping human tumors for the specific Ras mutation driving the cancer, so that the appropriate mutated Ras-bearing yeast will be administered.

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9.4 EXPANDING THE CATALOG OF YEAST-EXPRESSED TUMOR ANTIGENS Besides the GI-4000 yeast targeting individual Ras mutations, several GI-4000 yeast were engineered to express multiple Ras mutations as fusion proteins. All three different yeast strains currently being tested in the clinic harbor one of the three most common amino acid mutations at position 12 (G12V, G12 C or G12D), in addition to both Q61R and Q61L mutations described above. Other yeast strains have been engineered to express in one yeast the most common Ras mutations found in human cancer. These strains, namely, RastaFAR GI-4000 yeast, express fusion proteins with multiple cassettes of Ras polypeptide harboring different Ras mutations. Each of these polypeptide cassettes is sufficiently long so that epitopes for MHC-I and MHC-II presentation may be randomly generated in all different frames and lengths by antigen-presenting cells to support the diversity of HLA binding specificities found in humans. Table 9.1 lists additional yeast strains that have been engineered to express different tumor antigens. GI-3000 yeast express EGFR as an intracellular protein to target tumors overexpressing EGFR. The administration of GI-3000 yeast expressing EGFR has resulted in the breaking of immune tolerance to gliomas overexpressing EGFR (Lu Y et al, manuscript in preparation). The MUC1 and CEA proteins expressed in GI-6000 Tarmogens have been characterized as overexpressed differentiation or self-antigens in several ductal cancer types, including breast, pancreatic, and colorectal carcinomas. The apparent role of these tumor proteins in metastasis combined with the availability of well-defined transgenic systems featuring them makes these antigens especially attractive targets for yeast-based immunotherapy. Because of the apparent role of these tumor-associated proteins in metastasis, and

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Sources: (1) Lu Y et al, manuscript in preparation; (2) Borges V, Bellgrau D, Lu Y, Franzusoff A, unpublished observations; (3) Hodge JW, personal communication; (4) Fujita et al, in press. b Not tested to date.

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because of the availability of well-defined tumor models, including transgenic animal tumor models, these proteins are attractive target tumor antigens for yeast-based immunotherapy (Borges V, Bellgrau D, Lu Y, Franzusoff A, unpublished observations and Hodge JW, personal communication). The melanoma-associated MART1 protein expressed in GI-7000 yeast has elicited antigen-specific immune responses and tumor protection in transplant and transgenic animal tumor models [43]. Interestingly, since MART1 is not fundamental to the malignant phenotype (unlike EGFR or mutated Ras), eventually cells from treated animals shed the MART1 antigen under immune pressure [43], as would be predicted from the discussion above. The breadth of results demonstrating the immunogenicity of multiple antigens and tumor protection against these different targets illustrates the potential of yeast as a vector for therapeutic cancer vaccines. All of the tumor antigens expressed in yeast tested to date have been immunogenic and have promoted antitumor activity, even in transgenic or homologous model systems. Yeast-based immunotherapy has also been used to target immune responses against foreign antigens for infectious disease targets such as HIV, HCV, influenza, and fungi. A yeast Tarmogen, GI-5005, expressing an HCV NS3 and Core fusion protein is currently being tested in a phase II clinical trial. Further discussion of yeast-based immunotherapy for infectious disease indications is, however, beyond the scope of the current review.

9.5


Results with the urethane-induced tumor model showed that increasing the number of doses of yeast-based immunotherapy improves efficacy in animal tumor models. Studies performed with a variety of target antigens have revealed that increasing the amount of antigen expressed per yeast cell is also important for enhancing antigen-specific immune responses [21]. Thus, the number of yeast administered and the amount of antigen per yeast are two critical elements of yeast-mediated immune responses. In this section, the methods employed to increase the amount of heterologous antigen expressed while culturing yeast will be described. There are many parameters within vector design that likely impact heterologous protein expression. For this discussion, the focus will be on two key elements of yeast expression vector engineering: plasmid copy number and the yeast promoter used to express the heterologous protein. Controlling Plasmid Copy Number Gene expression cassettes may be integrated into the yeast genome as single or several units, or may be maintained as extrachromosomal, or episomal, plasmid elements at low, intermediate, or high copy number. Episomal plasmids are inherently unstable because of the extra metabolic cost required by the cell to replicate the episomal plasmid for cell division. To overcome this metabolic price tag, the

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plasmids encode an essential yeast gene product that allows the transfected yeast to survive and grow in media lacking specific nutrients. Thus, yeast that harbor genomic mutations in one or more biosynthetic gene products are considered auxotrophic for a particular biochemical pathway. The standard auxotrophies used for yeast engineering include mutations in the synthesis of individual amino acids, such as histidine (his3 mutations) or leucine (leu2 mutations), or pathways for nucleic acid precursors such as uridine (ura3 mutations) or adenine (ade2 mutations). A parent yeast that harbors these mutations can be “rescued” by addition of the end product to the media, or by introduction of the wild-type gene for that pathway on an episomal plasmid element to convert the cell from auxotrophy to prototrophy. Therefore, a plasmid engineered to express a heterologous protein also encodes the protrophic gene so that the plasmid will be retained by the cell under selective growth conditions. When the selection is no longer applied, then the yeast will no longer need to replicate the plasmid for survival and the episomal plasmid will be diluted by cell division until new daughter cells will be produced that no longer possess any episomal plasmids. The advantage of integrating the heterologous expression cassette into the yeast genome is the inherent stability of replicating chromosomes with each cell division. This means that yeast can be cultured in rich, nonselective media that allow for shorter generation times and the density of yeast number per liter of culture is typically an order of magnitude higher than that of yeast grown in minimal selective media. However, the number of copies of the desired gene product is then limited to the number of copies integrated into the yeast genome. Episomal plasmids may be engineered to encode replication regulatory sequences that dictate how many plasmid copies will be maintained in yeast cells with each cell division. One category of vectors encodes an yeast origin of DNA replication, the autonomously replicating sequence (ARS) plus a centromeric DNA element (CEN) that binds the yeast mitotic spindle, enabling accurate plasmid segregation. This class of plasmids is commonly referred to as “low copy” because accumulation is limited to approximately two to three copies per cell. The other major regulatory element, known as the origin of replication, is derived from a naturally occurring circular DNA found in yeast strains, and is also called the 2 -μm circle. This 2-μm origin or replication drives plasmid high-copy accumulation to ∼ 40–60 molecules per cell. These vectors maintain chromosome-like stability by signaling the recruitment of specialized plasmid-partitioning machinery [35,36]. Thus, the highest possible heterologous protein expression is typically achieved by employing 2-μm-based vectors for expression cassette gene copy numbers higher than those that could be obtained with either chromosomal integration or expression from the low-copy ARS -CEN -based vectors. One additional strategy for amplifying plasmid copy number is that a particular mutation in the LEU2 gene product, known as the leu2d mutation, may also be recruited as a selectable marker for replicating plasmids [37]. The defective leu2d protein is unable to support leucine biosynthesis unless the protein is present in high numbers in the cell (i.e., mass action to overcome the biochemical deficiency). Thus, yeast prototrophy achieved with the leu2d gene encoded on the vector results


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in very high copy numbers—100–200 copies per cell—when cells are grown in absence of added leucine to the culture media. In practice, though, selection using leu2d alone is more complex for initial clone isolation, presumably because of the requirement to achieve immediate high copy number of the episomal plasmids to support auxotrophic growth in the absence of added leucine. Hence, most vectors that utilize leu2d for selection also include another prototrophic selection, such as URA3 , so that the clones harboring the desired expression vector may be selected by growth in the absence of added uridine and once established, the copy number may be enhanced by eliminating added leucine to the culture media. Our studies have indicated that target genes expressed from URA3 + leu2d based vectors generate two- to five-fold more heterologous antigen in growth medium lacking both leucine and uracil than in medium lacking uracil alone (King T, Lu Y, Guo Z, Kelley V, Franzusoff A, unpublished observations). Promoter Choice: Inducible, Constitutive, or Repressible The second key element for engineering high-level heterologous protein expression is the choice of promoter for regulating transcription of the heterologous gene (factors that regulate the efficiency of translation initiation or transcription termination are not reviewed here). The type of promoter employed will be influenced by several interrelated factors, such as the need to control the timing of antigen expression, the impact of foreign protein expression on yeast cell health, and whether multiple antigens are produced from one or multiple plasmids within the yeast. The ability to control the rate of mRNA (and protein) accumulation is useful in cases where the antigen is toxic to the yeast cell and/or prone to aggregation. For such proteins, the investigator may need to have control over the rate and timing of mRNA synthesis. This may be achieved through the use of so-called rheostatic promoters—those that can be regulated by the addition of chemical compounds to the cell growth medium. By lowering the transcription rate (e.g., by adding a repressor compound) or initiating transcription late in the yeast culturing process, a heterologous protein that interferes with normal yeast growth may be better produced to higher levels, in addition to yielding higher-density cell harvest. For aggregation-prone proteins, slow or delayed synthesis is sometimes the key to avoiding these unproductive complexes because the protein molecules of interest are allowed to completely fold before encountering other partially folded proteins. From the process development standpoint, some of these controls are achieved by growing yeast at loweror higher-than-optimal temperatures. Wild-type yeast strains grow most rapidly at 30◦ C, but can be cultured at 4–37◦ C. A second advantage of rheostatic promoters is the ability to evaluate the consequences of antigen content independent of the number of yeast administered. Some useful rheostatic promoters for use in S. cerevisiae include those that can be induced (e.g., CUP1 ), repressed (MET25 ) or tuned in either direction (e.g., GAL1–10 , tetracycline-inducible or -repressible variants) [38–42]. The use of constitutive promoters may circumvent limitations of rheostatic promoters and can be useful for simpler proteins that are not prone to aggregation or

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that cause toxicity to the yeast. The compounds used to regulate rheostatic promoters are in some cases toxic and need to be removed from bulk yeast drug substance, which may complicate manufacturing and testing for clinical applications. These substances must be added at a defined point in the cell growth cycle, which imposes the need to closely monitor cell growth rate during the production run. The use of a constitutive promoter can mitigate or even bypass these problems because no promoter-activating compounds need to be added. Furthermore, transcription rates from constitutive promoters are frequently higher than those for rheostatic promoters, resulting in higher total levels of heterologous protein. Examples of strong constitutive promoters in S. cerevisiae include ADH1 (alcohol dehydrogenase), ENO2 (enolase), TEF1 , TEF2 (translation elongation factors 1 and 2), and GAPDH (glyceraldehyde-3-phosphate dehydrogenase). The endogenous products of these promoters are among the most abundant in the yeast cell under routine culture conditions (e.g., glucose plus nitrogen source). Promoters normally used to make components of the protein synthesis machinery are excellent choices for the present application because they generally drive transcription of essential, single-copy genes that must be activated to achieve protein synthesis. Thus, heterologous protein expression driven by a constitutive promoter that is normally used in ribosomal biogenesis will correlate directly with bulk cellular protein synthesis. Finally, promoter choice may be influenced by the need to create multiantigen products. Yeast have been engineered for immunotherapy to express two different antigens at the same or different levels, raising the question as to whether both genes should be expressed from the same or different promoters. The CUP1 promoter recruits a transcription factor (Ace1p) whose amount is limiting for maximal transcription of one mutated Ras protein, like that engineered for the GI-4000 strains. Overexpression of the ACE1 gene on a 2-μm-based plasmid was observed to result in doubling the quantity of CUP-driven expression of mutated Ras protein produced on copper addition (King T, Lu Y, Franzusoff A, unpublished observations). This example illustrates that a promoter can have limited capacity to drive transcription because of a limited supply of a promoter-specific transcription factor (or factors). If two different antigens should be highly expressed, the best expression strategy may therefore be to utilize two strong promoters, such as TEF2 and GAPDH , that use independent regulatory factors. This discussion illustrates that there are numerous ways to express tumor antigens in yeast. It is also important to point out that antigens may be localized to different compartments in yeast and still be immunogenic, specifically, as cytosolic proteins or membrane proteins, and even secreted into the periplasm, as long as the protein remains behind the cell wall or is somehow retained by yeast after administration [6]. Success can be achieved with various strategies, and the optimal system will be empirical, depending on the number of tumor antigens being expressed and how their biochemical properties affect protein accumulation and yeast cell metabolism. In the end, the optimal yeast expression system will result in high-level heterologous protein levels with a simple manufacturing protocol that does not involve many post-culturing steps to reduce the potential for toxic by-products of manufacturing.


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Parameters of Dose Responses with Yeast-Based Immunotherapy Four parameters of dosing with recombinant yeast have been investigated: (1) the amount of antigen expressed per yeast cell, (2) the number of yeast administered per dose, (3) the number of doses administered over time, and (4) the route of administration. The importance of these four parameters to efficacy of yeast-based immunotherapy have been elucidated with in vitro and in vivo studies of T-cell-dependent immune responses and tumor protection, respectively [21]. These studies were performed with yeast expressing several different tumor or infectious disease antigens, so the identity of the antigen does not seem to influence the reproducibility of observations related to the parameters of dosing efficacy. Increasing the number of yeast and the amount of antigen expressed per yeast appears to improve the relative abundance of CTLs in immunized animals, particularly for foreign antigens and mutated self-antigens. As described in Section 9.4, the amount of antigen per yeast may be modulated by using different promoters and plasmids to express the same antigen at different levels in different yeast strains, or to modulate the activity of an inducible promoter by adjusting the amount of inducer (e.g., copper) added to the cultures for activating antigen expression in yeast. Thus, for foreign antigens (HIV-Gag, HCV NS3 and core proteins, influenza HA and M1 antigens) and mutated self-antigens (mutated Ras), a “more is better” paradigm appears to be operative, namely, more yeast administered plus more antigen expressed per yeast. Immune efficacy with recombinant yeast expressing lower amounts of antigen may still be achieved by increasing the number of yeast administered per dose. Similarly, fewer yeast are needed to exceed the threshold of immune efficacy if the heterologous antigen is expressed at very high levels in yeast, specifically, when the antigen is expressed as high as 5–10% of total yeast cell protein [21] (Franzusoff A, Duke R, unpublished observations). Surprisingly, for targeting self-antigens such as wild-type EGFR and CEA, the results from preliminary studies suggest that the administration of fewer yeast, albeit with high levels of heterologous antigen expressed per yeast, may be more efficacious than dosing with higher numbers of yeast (Hodge JW, personal communication; Lu Y, Franzusoff A, manuscript in preparation). The reasons for this alternate paradigm are currently being investigated. Activating T cell responses by immunization via multiple different routes of administration has been studied with in vitro and in vivo models and with different yeast-expressed antigens. Immune responses have been assessed by multiple parameters, namely, by lymphocyte proliferation, CTL, ELISpot (enzyme-linked immunospot assay) and intracellular cytokine secretion assays, to compare the impact of administering yeast by subcutaneous, intranasal, oral, intraperitoneal, and intravenous routes. Each of these routes of administration were effective for activating antigen-specific immune responses with recombinant yeast. Interestingly, more profound differences were observed when animals were challenged with tumors implanted in different locations. Subcutaneous and intranasal administration of yeast-based immunotherapy were equally effective against challenge

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with tumors implanted subcutaneously. However, when tumors were implanted intracranially, administering recombinant yeast by the intranasal and oral routes achieved significantly better protection against tumor challenge than did subcutaneous dosing (Lu Y et al, manuscript in preparation). Thus, the choice of administration route may be important for specific disease indications, since T cells that become activated by one route of administration are likely to recirculate in the related lymphoid tissues for surveying the appearance of target antigens presented by diseased cells. For instance, the intranasal and oral routes are effective for activating mucosal-tissue-associated lymphoid surveillance. Hence, some diseases that are associated with mucosal tissues, such as lungs, intestines as well as head and neck cancers, may receive more attention of T cells activated by the mucosal route. The relative benefit of route of administration for specific indications is admittedly speculative. In summary, yeast-based immunotherapy surprisingly exhibits dose–response parameters commonly associated with dosing small-molecule drugs. In this regard, the amount of antigen in the yeast, the number of yeast given, the number and schedule of immunizations, and the route of administration are all factors contributing to the extent of the immune response and its effectiveness in a given model system. These factors may contribute different levels of importance depending on the antigen and model used, and thus represent an empirical guide for testing. These preclinical findings continue to be evaluated with more antigens engineered into the yeast and with cancer models of spontaneously induced tumors or infectious diseases that involve different organs. The more conclusive tests of these hypotheses will require randomized controlled trials with patients harboring these different diseases.

9.6 PRECLINICAL SAFETY STUDIES AND CLINICAL TRIALS IN CANCER WITH YEAST-BASED IMMUNOTHERAPY Preclinical Safety Studies with Yeast-Based Immunotherapeutics Yeast-based vaccines expressing three different antigens have been tested in five separate toxicity studies in rabbits conducted under Good Laboratory Practices (GLP). Rabbits were injected subcutaneously with 0.5–100 YU per injection site, and up to 13 weekly injections before histopathological analysis. Aside from occasional minor injection site reactions that resolved after 2 weeks’ recovery, none of the rabbits showed signs of abnormality, aside from increased levels of circulating neutrophils. The equivalence of the safety profile from the five GLP toxicity studies implies that the yeast vector, not the expressed antigen, has the greatest influence on the reactions to immunization. Indeed, the investigational new drug (IND) application for the three GI-4000 series of products that are being tested in the clinic in patients with cancers harboring mutated Ras was filed with a single chemistry manufacturing and controls (CMC) section, by agreement with the FDA. However, because of the mechanism of action elucidated by the study with

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urethane-induced tumors in mice (described in Section 9.2), only one of the three GI-4000 yeast is administered to cancer patients. The selection of which GI-4000 yeast to administer is governed by the identification of the Ras mutation genotype in the patient’s tumor to match the yeast expressing the relevant mutated Ras polypeptide. GI-4000-01 Phase I Safety and Immunogenicity in Metastatic Cancer In a phase I clinical trial of GI-4000 series yeast, 33 subjects with advanced pancreatic or colorectal cancer were enrolled. Greater than 90% of the patients enrolled in the trial had metastatic disease at the time of enrollment, and subjects had received an average of three previous therapy regimens prior to participation in the GI-4000-01 trial. The design for this “first time in human” study was a dose escalation trial, in which cohorts received either 0.1, 1, 5, 10, 20, or 40 YU of the mutation-matched GI-4000 yeast administered subcutaneously for 5 weekly doses. Subjects were monitored for safety, local injection site reactions, complement cascade activation post-injection, and antigen-specific immunoloGI-c responses. No dose-limiting local or systemic toxicities were observed at any of the doses tested. The majority of subjects exhibited antigen-specific responses as demonstrated by lymphocyte proliferation and/or intracellular cytokine staining assays. Several subjects continue to be followed for survival. A manuscript describing the results of this study is in preparation. GI-4000-02 Phase II Testing in Resected Pancreas Cancer GI-4000-02 is a multicenter, double-blind, placebo-controlled, adjuvant trial comparing GI-4000 in combination with a gemcitabine regimen versus a gemcitabine regimen alone in patients with successfully resected ras-mutation-positive pancreas cancer. Another enrollment criterion is that the surgery have a tumor resection status of R0 or R1, indicating that post-resection, the pathology shows a resection margin that is free of even microscopic disease (R0), or that there is evidence of microscopic disease at the resection margin, but no macroscopic disease (R1). Subjects will receive three doses of GI-4000 yeast after resection, but prior to initiation of gemcitabine. Monthly doses of GI-4000 or saline placebo are administered after the start of gemcitabine during the drug holiday between gemcitabine cycles. The primary endpoint of the trial is median overall survival at 15 months after resection. 9.7

CONCLUSIONS

This chapter summarizes key aspects of using the recombinant yeast Tarmogen platform technology for cancer immunotherapy. The danger signals from recombinant yeast immunization trigger a multifactorial innate immune response. This

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innate immune response profoundly impacts and activates the antigen-presenting cells, which promotes specific T cell responses to the yeast-delivered tumor antigen. The yeast-based immunotherapy platform has been used for immune responses against a variety of foreign, mutated, or self-antigens. The expression of polypeptide tumor antigens in yeast promotes the processing of a comprehensive assortment of peptides in these yeast-activated APCs. The delivery of this broad catalog of peptides into antigen presentation pathways increases the probability that MHC receptors from diverse HLA genotypes will be able to bind peptides derived from yeast-expressed antigens. As a result, even tumors bearing single amino acid mutations in Ras, or overexpressing self-antigens, are capable of being targeted for destruction by yeast-mediated immune responses in a broad range of immunized hosts. The specific targeting and ablation of tumor cells that is highly desirable for the immune protection against cancer may be achieved without off-target side effects. Through the many studies with yeast-based immunotherapy in animals and the phase I clinical trial in cancer and the phase Ib trial in chronic hepatitis C infection, numerous significant (including some surprising) observations about the use of recombinant yeast for activating innate and antigen-specific immune responses have emerged: Antigen delivery and yeast-mediated immune responses •

Dosing with recombinant yeast drives a Th1-type cytokine response profile, including TNFα, GM-CSF, and IFNγ. • Recruitment and activation of APCs does not require the addition of artificial adjuvants. • Yeast-expressed antigens are delivered into the MHC-I pathway by nonclassical “cross-presentation”, in addition to classical MHC-II presentation of yeast-delivered antigens. • Orders-of-magnitude improvement in antigen-specific CD8+ T cell responses when target antigen is expressed in yeast compared to mixing soluble protein with empty yeast. Vaccine efficacy • • •

•

•

Recombinant yeast are immunogenic by multiple routes of administration. Yeast-based immunotherapy is able to break immune tolerance to self-antigens. The potential for administering “cocktails” of yeast expressing different antigens may be adopted for customizing the immunotherapy according to the target tumor genotype. The yeast may be immunized repeatedly without eliciting neutralizing antibodies that interfere with improvements in antigen-specific immune response. The clean safety profile observed with dosing recombinant yeast in animals and humans.

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Vaccine manufacturing •

Heat-killing yeast does not destroy the immunogenicity of yeast-expressed antigens (although this must be confirmed for each new antigen tested). • The simplicity and scalability of the Tarmogen manufacturing process. In closing, immunotherapy represents an attractive strategy for the treatment of cancer. Nevertheless, the challenge of dealing with high-burden tumors in late-stage cancer patients should not be underestimated. The phase II trial design that we have adopted for testing yeast-based immunotherapy in pancreas cancer specifically avoids the complications of high-burden cancer (see Section 9.6, on GI-4000-02 phase II testing in resected pancreas cancer). The range of potential obstacles that may emerge with treating high-burden tumors include immunosuppressive factors such as TGFβ, amplification of regulatory T cells, as well as the potential for reduced numbers of nascent immune cells to promote antigen-specific immune responses in patients with advanced cancer. Fortunately, cancer immunotherapy may be combined with other strategies that target these hurdles contributed by high tumor burden, such as targeted therapy with monoclonal antibodies or small-molecule drugs and chemotherapeutic regimens. We are currently exploring the impact of combining yeast-based immunotherapy with approaches that are predicted to reduce or overcome these hurdles.

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PART IV CELL-BASED THERAPY: USING CANCER CELLS AS A MEANS TO INDUCE SPECIFIC TUMOR IMMUNITY

10 ALLOGENEIC WHOLE-CELL VACCINES John Copier and Angus Dalgleish

10.1

INTRODUCTION

There is evidence that allogeneic whole-cell vaccines can raise immune responses and have clinical efficacy in prostate cancer and in melanoma [1,2]. Much effort has been applied to the discovery of tumor-specific antigens, resulting in tried and tested vaccination strategies involving peptide, whole protein, and cDNA. In contrast, whole-tumor cells have been used in a “polyvalent” approach that does not require knowledge of tumor antigens but instead assumes that target antigens are present in the vaccine cells themselves. In this chapter we will deal with the mechanisms by which whole-cell vaccines work and discuss the ways in which modification of vaccine cells may improve their efficacy. Particular emphasis will be given to those vaccines with demonstrable clinical outcomes.

Cancer Vaccines and Tumor Immunity Edited by Rimas Orentas, James W. Hodge, and Bryon D. Johnson Copyright © 2008 John Wiley & Sons, Inc. 153

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10.2 HOW WHOLE-CELL VACCINES ELICIT EFFECTIVE T CELL RESPONSES Preclinical studies in mice have demonstrated robust antitumor responses to wholecell vaccines that are dependent on a variety of immunologic mechanisms, including T cell (both CD4+ and CD8+ ) and NK responses, and the presence of macrophages and eosinophils depending on the formulation of the vaccine [3–10]. How these vaccines work is poorly defined and remains an area of controversy. The majority of studies indicate a substantial requirement for cytotoxic T cell responses that are elicited through one or both of the following mechanisms: either (1) the vaccine cells directly prime T cells or (2) vaccine antigens are taken up by professional antigen-presenting cells (APCs), which then stimulate a T cell response, an effect known as cross-priming. Evidence for direct priming is scant, but a few studies have indicated that access of tumor cells to the T cell areas of the lymph node are a requirement for tumor rejection, indicating the likelihood of a direct interaction between the vaccine and the T cells [11–13]. Dendritic cells are capable of taking up and presenting tumor antigens [14,15]. Moreover, several studies have shown that cross-priming through presentation on host APCs is of greater importance than direct priming by the tumor cell since a match between the tumor MHC and the host was irrelevant [15,16]. Whether direct priming or cross-priming is the central mechanism for stimulating cytotoxic responses against whole-tumor vaccines remains controversial, and at least one study has suggested that both mechanisms are required [13]. It is clear that in some tumor models depletion of CD4+ cells abrogates the responses to vaccination. Although initial data implied that MHC class II–positive tumor cell vaccines directly primed CD4+ T cells [17,18], more recent data show that dendritic cells are absolutely required for this process [19]. Furthermore, there is startling evidence that MHC class I and class II molecules can be transferred (presumably along with their peptides) from the tumor vaccine to dendritic cells, a mechanism called “cross-dressing” [19].

10.3

ADVANTAGES OF WHOLE-CELL VACCINES

There are a number of advantages to using whole-cell vaccines. As mentioned above, there is no specific requirement for knowledge of the nature of the target antigen. In addition, cellular vaccines can be modified in ways that modulate immune responses (see discussion below). The polyvalent approach assumes that vaccine cells have an antigenic profile identical to, or overlapping with, the patient’s tumor. In the case of autologous tumor cell vaccines, prepared directly from resected tumor tissue, this is probably the case, and clinical benefit has been shown using such vaccines in randomized trials of colorectal and renal cancers [20,21]. However, autologous vaccines have a number of disadvantages when translated to the clinic. Resected autologous tumor’s must provide enough cells for a vaccine schedule, and where this is not possible, they must be amenable to culture, which is often not

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the case. Moreover, lengthy culture periods can allow sufficient time for a patient’s disease to progress. Since allogeneic cell lines are already established in culture, this is not a problem and vaccine is relatively easy to prepare. Furthermore, allogeneic cells can be prepared as a “modified” vaccine (i.e., transduced to express cytokines or costimulatory factors). Autologous cell lines modified in this way are difficult to establish and similarly have the problem of lengthy culture time. In order for allogeneic vaccines to work, it is necessary for them to share an antigenic profile with the patient’s tumor. Several preclinical studies have demonstrated that selection of the source of tumor cell line is important to determine its effectiveness as a vaccine. Allogeneic cell vaccination was successful in a B16-F10 melanoma model when allogeneic cells of a melanoma background were used but not when the vaccinating cells were derived from an irrelevant carcinoma [22,23]. However, not all allogeneic melanoma cell lines were able to protect against challenge with B16-F10 [22]. These studies suggest that very careful selection of tumor cell lines must be made in order to match the patient with the vaccine. Cell lines derived from secondary tumor sites require careful screening, as it has been demonstrated that expression levels of known tumor antigens vary considerably depending on their origin [24,25]. Furthermore, there are variations in other factors that may influence immunogenicity, such as expression of LMP and TAP, although treatment with IFNγ was shown to upregulate these molecules, indicating that the immunogenicity of the cell lines can be manipulated in vitro. To ensure an adequate antigenic profile, most cellular vaccines incorporate several different cell lines from the same background as the patient’s tumor [1,2].

10.4

EFFECTIVENESS OF ALLOGENEIC WHOLE-CELL VACCINATION

Proof of principle for the effectiveness of allogeneic whole-cell vaccination has been established in mouse tumor models where both prophylactic protection against tumor challenge and the presence of tumor-specific CTLs was demonstrated [22,26,27]. In a rat Lobund–Wistar model of prostate cancer, prophylactic vaccination with an allogeneic cell vaccine was effective in protecting 80% of the treatment group against challenge with a PAIII tumor [28]. When this was repeated in a Copenhagen rat model (where the cancer was poorly immunogenic and more aggressive), no protection was elicited by treatment with allogeneic vaccine. However, this and other studies have shown that addition of adjuvant to ineffective allogeneic vaccines leads to protection against tumor challenge [27,28]. Clinical trials have been quick to follow, and while encouraging clinical results have been shown in melanoma and prostate cancer, there remain difficulties in monitoring immunologic responses to such vaccines. Unlike monovalent vaccines, where the antigen or even the epitope is known, polyvalent vaccines rely on unknown antigens. It is therefore difficult to apply immunologic tests that measure the specificity of the response. Most investigators look for DTH responses (against the vaccine), CTL/IFNγ responses or cytokine responses (raised in vitro

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against either the vaccine or autologous tumor). However, it is difficult to interpret these data unless there is a clear correlation with a clinical response. A vaccine composed of three allogeneic cell lines (OnyCap23, LncaP, and P4E6) has been used in clinical trials for the treatment of hormone-resistant prostate cancer with encouraging results [2]. Patients treated with the vaccine had a median time to progression of 58 weeks, compared to 26 weeks for historical controls, and there were prolonged decreases in PSAV (a prognostic biomarker used to assess the response of prostate cancer to therapy) in 11 of 26 patients. Immunologic responses were monitored and analyzed using artificial neural networking (a powerful multiparametric analytic tool that can be used when standard statistical methods fail to show patterns in the data). This analysis revealed that positive clinical outcome correlated with a reduced PSAV and a TH 1 response compared to nonresponsive patients who had a mixed TH 1/TH 2 response. A phase IIB trial began in August 2006. Clinical trials in melanoma have also been encouraging. Vilella and coworkers used a mixture of 10 allogeneic melanoma cell lines in combination with BCG (as an adjuvant) to vaccinate metastatic melanoma patients and showed clinical benefit in 6 of 23 patients: three complete responses, two partial responses, and one mixed response. Patients had a median survival of 20.2 months [29]. A series of trials for Canvaxin (a mixture of three allogeneic melanoma cell lines plus BCG) showed clinical benefit and immunologic responses in advanced melanoma patients. In phase II trials the vaccine significantly prolonged survival in the treatment arm when compared with a nonvaccinated control arm [30,31]. A preexisting TH 2 cytokine profile in peripheral blood CD4+ T cells was associated with better prognosis following vaccination and, moreover, DTH and humoral responses to vaccine were shown to correlate with clinical outcome [30,31]. However, a phase III trial in which Canvaxin was compared to a control arm (BCG alone) was stopped early as there was no evidence that the two arms were diverging. The BCG arm itself did better than expected and may account for the differences between this and earlier trials. Despite the failure of this trial, earlier results were convincing enough to warrant some optimism since differences between the phase II and phase III trial results may be due to differences in the trial design or patient selection [32]. Moreover, melanoma studies often result in disappointing stage III results after encouraging phase II trials, perhaps reflecting the chaotic nature of melanoma, which is characterized by rapid division and widespread metastasis.

10.5 MODIFICATION OF ALLOGENEIC CELL VACCINES WITH COSTIMULATORY MOLECULES Tumor cells are generally thought to be poorly immunogenic, although immunogenicity varies considerably between tumors of different backgrounds. A variety of strategies have arisen that are aimed at improving the immunogenicity of the vaccine including modification of vaccine cells to express costimulatory molecules or cytokines. Modification of allogeneic cell lines is often difficult as transfection

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often fails and it is necessary to use viral vectors to introduce the relevant cDNA, which itself carries certain implied health risks to patients. Nevertheless, a certain degree of success has been obtained in preclinical and clinical studies using modified allogeneic cells as cancer vaccines. Poor immunogenicity is due at least partly to the heterogeneous expression of MHC and costimulatory molecules [33,34]. The absence of costimulation raised concerns that vaccine cells might induce tolerance, and consequently the introduction of B7.1/CD80 costimulatory molecules into vaccine cells was investigated in preclinical models. Several studies showed that expression of CD80 on autologous vaccine cells elicited CTL responses and improved protection against tumor challenge [35–38]. However, results from one study suggested that CD80 expression is effective only when the vaccine cells are themselves immunogenic [39]. In allogeneic models effective protection against tumor was also demonstrated [40]. Studies using human cells have shown that introduction of costimulatory molecules results in a large alloresponse, but despite this finding, tumor-specific CTLs could also be demonstrated [41,42]. Furthermore, HLA-A2+ allogeneic breast cancer cells modified to express CD80 were shown to stimulate CD8+ T cell responses from PBMCs of an HLA-A2+ patient [43]. In a phase I clinical trial to treat 19 patients with relapsed metastatic NSCLC, an allogeneic vaccine modified to express CD80 and a patient-matched HLA-A molecule (either A1 or A2) was used [44]. In the majority of patients, IFNγ release from CD8+ cells was seen (as measured by ELISpot) in response to stimulation with mock transfected vaccine cells (CD80− /HLA-A1/A2− ). Patients had a 32% response rate (one partial response and five stable disease) with a median overall survival of 18 months compared to the previously reported median survival of less than one year for metastatic lung cancer. The apparent effectiveness of modification of vaccine cells with costimulatory molecules raises further questions concerning the mechanism by which whole-cell vaccines stimulate T cell responses. Introduction of costimulatory molecules into vaccine cells presumes that direct priming forms a component of the mechanism for T cell stimulation. Work by Cayeux and colleagues showed that, in mouse models, allogeneic cellular vaccines caused rejection of tumor via a predominantly cross-presentation pathway [40]. However, direct priming occurred when CD80 was introduced into the vaccine, suggesting that introduction of costimulation may influence the mechanism of T cell stimulation. It is interesting to note that many allogeneic cell vaccines used in humans are matched for at least one HLA allele, leaving the possibility that such vaccines work, at least in part, through direct presentation of antigen. 10.6 MODIFICATION OF ALLOGENEIC VACCINES TO SECRETE CYTOKINES Cytokines have been used to treat advanced cancers, including renal cell carcinoma and metastatic melanoma with some success [45,46], but systemic administration

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is associated with marked toxicity. Recombinant cytokines expressed by modified tumor cells can be used to target the cytokine to tumor or vaccination sites where they can induce a proinflammatory environment, with the additional benefit of lowering the levels of circulating cytokine and therefore circumventing the toxicity related to high systemic doses. Cytokine-secreting tumor cells have been tested extensively in mouse models for their efficacy in reducing tumorigenicity, protecting against tumor challenge and raising immune responses. To date a range of cytokines, including IL-2 [6,47,48], IL-4 [8,49,50], IFNγ [51,52], GM-CSF [53], IL-12 [54,55], IL-15 [56], IL-21 [57–60], IL-23 [61–63], and IL-27 [64–66], have been investigated in models of autologous tumor cell vaccination with a considerable degree of success. Of these only IL-2 and GM-CSF have been shown to have potential in allogeneic tumor cell vaccination models. Interleukin 2 is produced by CD4+ T cells of the TH 1 type. It supports recruitment, differentiation, and proliferation of T and NK cells and is frequently used systemically for the treatment of renal cell carcinoma and melanoma. In autologous IL-2-secreting cell vaccination models there is a requirement for CD8+ T cells and/or NK cells to establish effective protection against tumor challenge [4,5,47]. In a study where allogeneic cell vaccines were shown to be protective against tumor challenge in a prophylactic treatment model, IL-2-secreting vaccine was more effective than unmodified cells [67]. Furthermore, vaccination with IL-2-secreting allogeneic cells gave rise to a cytotoxic response that recognized syngeneic tumor. This study also showed that a mixture of IL-2-secreting autologous cells and allogeneic cells gave complete protection in a therapeutic model. In human studies limited immunologic responses have been demonstrated after IL-2-modified allogeneic cell vaccination. Two studies in melanoma patients showed that CD8+ T cell responses could be mounted against vaccine cells in a small proportion of the patients [68,69]. Another study demonstrated that IgG responses could be stimulated using the allogeneic melanoma cell line Mel 4932 [70]. These sera stained autologous tumor and could elicit ADCC responses against Mel 4932, although no killing of autologous tumour was seen. Trials using allogeneic IL-2-secreting tumors have generally been clinically disappointing [68,69]. However, one notable study showed clinical benefit when patients were vaccinated with a mixture of allogeneic IL-2-secreting cells and autologous cells [71]. Of 30 patients with metastatic RCC, objective responses were seen in 14 (1 complete response, 4 partial responses, 9 with stable disease). There was improved median survival in the vaccination group compared to control subjects (18.9 months and 12.2 months, respectively; p = .01). Although GM-CSF has highly pleiotropic effects, it is involved principally in stimulation of proliferation and activation of granulocytes and monocytes. GM-CSF stimulates dendritic cell maturation and is able to augment APC function [72]. Protective immunity is induced by vaccination with allogeneic GM-CSF-secreting cells, and stimulation of cytotoxic responses to tumor cells has been demonstrated [26,73,74]. Furthermore, antitumour responses were shown to be dependent on both CD4+ and CD8+ T cell subsets. GM-CSF-secreting tumor cell vaccines appear to elicit responses through a cross-priming mechanism [40], which is consistent

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with the data showing that GM-CSF secreting allogeneic cell vaccine leads to an accumulation of dendritic cells at the vaccination site [75], a process that is dependent on MIP-1α (a chemokine secreted by macrophages). In clinical trials a GM-CSF-secreting allogeneic vaccine composed of two pancreatic cell lines (PANC-10.05 and-6.03) exhibited a trend toward disease free survival in patients with pancreatic adenocarcinoma, although patient numbers were too small to make this study statistically significant [76]. More recent studies have used a combination of GM-CSF-secreting cells (PC-3 and LNCaP) for the treatment of advanced hormone-resistant prostate cancer. Presentations at ASCO suggest that in 7 of 10 patients there was a trend toward increased survival [77]. Several other phase I/II trials using this allogeneic vaccine are ongoing for prostate cancer. These studies demonstrate that, in principle, immunologic responses can be raised to IL-2- and GM-CSF-secreting allogeneic tumors and that some clinical benefit can be demonstrated. However, the clinical efficacy of these vaccines (and in particular GM-CSF) have not borne out the promise of preclinical trials, and further work is required to optimize these treatments for humans.

10.7

FUTURE RESEARCH

Improvements in our understanding of the underlying biology of tumours and the nature of antitumour responses may help us to improve the efficacy of whole-cell vaccines. It is well known that tumors provide an immunomodulatory environment in which T cell responses may be suppressed. One key factor in the modulation of antitumor responses is the presence of T-regulatory cells (Treg), a subset of CD4+ T cells that suppress immune responses and are found in many tumor environments [78]. Ablation of Tregs prior to immunotherapy has resulted in improved protection indicating that antitumour T cell responses are effective when not suppressed by Tregs. Several preclinical studies have shown that treatments that affect immunomodulation by Tregs, such as CTLA-4 blockade or Treg depletion, result in improved effectiveness of cytokine-secreting tumor cell vaccines [59,62,79,80]. These studies have been performed with autologous vaccines, and work has yet to be done to determine the efficacy of Treg modulation in combination with allogeneic vaccines. There is a growing movement in the field of immunotherapy toward the combination of vaccines with other forms of intervention, including standard chemotherapy and radiotherapy. GM-CSF-secreting autologous vaccines, have been studied in combination with chemotherapy, principally doxorubicin, docetaxel, paclitaxel, and cyclophosphamide [81–83]. These preclinical studies demonstrated improved protection against tumor challenge when vaccine was administered in combination with chemotherapy, but that efficacy was dependent on correct timing of administration of the drug and the vaccine. Further work is required to demonstrate the effectiveness of vaccine–chemotherapy combinations in the allogeneic setting. Clinical

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trials are currently under way to test the allogeneic GM-CSF vaccines in combination with docetaxel for hormone-resistant prostate cancer (GVAX, Cellgenesys) and with cyclophosphamide and doxorubicin. 10.8

CONCLUSIONS

Whole-cell allogeneic vaccines have shown promising results in preclinical and clinical settings. Other polyvalent vaccine strategies, beyond the scope of this chapter, using allogeneic tumor cell lysates have been more clinically successful in prostate cancer and melanoma [20,84], suggesting that the antigens required to raise an antitumour response are present but that the method of presentation of the vaccine is important. Furthermore, dendritic cells loaded in vitro with whole allogeneic cell lysates of tumor cells have been investigated in melanoma with some encouraging results [85,86]. These strategies effectively mimic the first steps of cross-presentation that occur in vivo and allow tumor antigen presentation by professional APCs. However, the requirements for optimal loading of dendritic cells with lysate remains elusive. Ultimately, in vitro loading of dendritic cells will provide indications of the requirements for cross-priming in vivo and give clues to the improvement of whole-cell vaccines. Further work is required to determine the best strategy for allogeneic whole-cell vaccination. Parameters such as number of cells, route of injection, timing of vaccinations, and presence of adjuvant still require optimization. The potential for use in combination with other therapies adds a further layer of complexity. However, results from unmodified allogeneic cell vaccines are very encouraging for prostate cancer [2] and possibly melanoma [1]. Acknowledgment Angus Dalgleish and John Copier are supported by funds from the Cancer Vaccine Institute. REFERENCES 1. Hsueh EC, Morton DL: Antigen-based immunotherapy of melanoma: Canvaxin therapeutic polyvalent cancer vaccine, Semin Cancer Biol 13(6):401–407 (2003). 2. Michael A, Ball G, Quatan N, Wushishi F, Russell N, Whelan J, Chakraborty P, Leader D, Whelan M, Pandha H: Delayed disease progression after allogeneic cell vaccination in hormone-resistant prostate cancer and correlation with immunologic variables, Clin Cancer Res 11(12):4469–4478 (2005). 3. Bannerji R, Arroyo CD, Cordon-Cardo C, Gilboa E: The role of IL-2 secreted from genetically modified tumor cells in the establishment of antitumor immunity, J Immunol 152(5):2324–2332 (1994).

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11 JUMP-STARTING TUMOR IMMUNITY WITH BREAST CANCER THERAPEUTICS Leisha A. Emens

11.1

INTRODUCTION

Breast cancer stands out among the solid tumors as a model for advances in cancer care. The past few decades have produced multiple incremental improvements in local management with surgery and radiation, and in systemic treatment with endocrine manipulation and chemotherapy. In the aggregate, these improvements have decreased the morbidity and mortality of breast cancer in a clinically meaningful way, but up to 40% of patients diagnosed with the disease continue to ultimately succumb to it [1]. More recently, the genomic revolution has opened the door for designing therapy individualized to the patient’s tumor biology as reflected by gene expression patterns rather than anatomic staging. This advance will build on the longstanding success of tamoxifen as an effective treatment for tumors that express the estrogen receptor α (ERα), as it will facilitate the incorporation of novel therapeutics that target the critical regulatory pathways underlying breast cancer biology. The clinical success of the HER2/neu-specific monoclonal antibody trastuzumab illustrates the power of this customized approach to disease Cancer Vaccines and Tumor Immunity Edited by Rimas Orentas, James W. Hodge, and Bryon D. Johnson Copyright © 2008 John Wiley & Sons, Inc. 169

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management [2]. Despite considerable progress and the promise of novel targeted therapies, fixed drug resistance remains a major limit to the efficacy of traditional drug-based systemic breast cancer therapy, arguing for an innovative approach to the management of mammary cancer. Tumor vaccines offer a unique alternative that recruits the patient’s own immune system to the therapeutic effort, altering the host–tumor relationship rather than directly targeting the tumor itself. A number of breast cancer vaccines have already been tested in the clinic, with some evidence of vaccine-induced immunity [3]. However, there has been little evidence of clinical benefit to date. Multiple factors likely explain this lack of success: (1) breast cancer vaccines have frequently been tested in advanced disease, where the disease overwhelms the immune response by both its physical magnitude and tumor-associated networks of immune suppression; (2) most breast cancer vaccine formulations are relatively weak immunogens, with suboptimal platforms, tumor antigens, or both; and (3) many trials have simply added a breast cancer vaccine to standard systemic therapy with little regard for how they might interact. Consideration of each of these factors should facilitate the future design of robust clinical vaccine trials that yield important mechanistic and clinical information, thereby hastening the development of breast cancer vaccines and bringing them to clinical fruition. 11.2

BARRIERS TO EFFECTIVE BREAST TUMOR IMMUNITY

As suggested above, barriers to effective breast tumor immunity include the sheer physical burden of tumor cells in advanced disease, well-established mechanisms of systemic immune tolerance, and immunosuppressive regulatory pathways specific to the tumor microenvironment itself. Therefore, strategies for combining breast cancer vaccines with standard cancer therapeutics can be developed that minimize the impact of tumor burden by cytoreduction, lift layers of systemic immune tolerance to allow tumor immunity to develop, augment T cell activation, and groom the tumor microenvironment to facilitate the activity of infiltrating immune effectors. Ideally, combinatorial immunotherapy will incorporate each of these strategies to produce a synergistic antitumor response that is effective and uniquely durable by virtue of the immunologic memory response. 11.3

CHEMOTHERAPY

Tumor Immunity and Cytoreduction Therapeutic doses of chemotherapy can enhance the activity of tumor vaccines in at least three ways: (1) by achieving a state of minimal residual disease prior to vaccination, (2) by augmenting vaccine-induced immunity through treatment-related apoptosis, or (3) by skewing the T cell repertoire during lymphopenia-induced homeostatic proliferation.

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The magnitude of both the spontaneous and therapeutic antitumor immune response is frequently outmatched by the sheer physical burden of the tumor cells it must contend with, particularly in the setting of advanced, disseminated disease [4]. In this case, it may be prudent to use standard cancer therapy (including surgery, radiation, and chemotherapy) to debulk existing disease, thereby achieving a state of minimal residual disease prior to or after therapy with a tumor vaccine. In this case, cytolytic doses of chemotherapy are used. Whether the impact of chemotherapy on the vaccine-induced immune response is positive or negative depends on the drugs, the dose given, and the timing of chemotherapy in relation to immune-based therapy [5]. At least two studies have shown that standard-dose chemotherapy can inhibit the activity of tumor vaccines if given in close proximity or as multiple cycles [6–8]. A distinct study showed that the vaccine-induced immune response can be restored with appropriate vaccine boosting schedules [6,8]. Interestingly, several studies have shown that standard-dose chemotherapy given after vaccination in patients with various types of advanced cancer correlates with clinical benefit and (in some cases) with enhanced antitumor immunity [9,10]. This observation is likely due to the boosting of preexisting immunity by chemotherapy-induced tumor apoptosis. In human breast cancer, neoadjuvant paclitaxel (PTX) chemotherapy induces both tumor apoptosis and the accumulation of tumor-infiltrating lymphocytes (TILs) at the tumor site, with the extent of first-dose apoptosis predicting both increased numbers of TIL and clinical benefit [11]. Finally, vaccinating during lymphopenia, such as occurs during bone marrow transplantation or after aggressive chemotherapy, can direct the reestablished T cell repertoire toward a desired antigenic specificity [12–15]. This concept has been evaluated in clinical trials testing the adoptive transfer of T cells after myeloablative chemotherapy [16–18], with a correlation seen between tumor regression and significant levels of peripheral tumor-specific T cells in some cases [17,19–21]. Immune Tolerance and Activation Chemotherapeutic agents can also modulate immune tolerance in novel ways (Fig. 11.1), depending on the dose and timing in relation to an antigen exposure [5,22,23]. Cyclophosphamide (CY) and doxorubicin (DOX) have been the mainstay of adjuvant chemotherapy for breast cancer. Both can modulate immunity, particularly when given at low doses. For example, CY given at low doses 1–3 days prior to antigen exposure can break immune tolerance, facilitating both humoral and cellular immunity. Conversely, CY given at the time of or after antigen exposure induces immune tolerance. CY augments tumor immunity by eliminating the influence of CD4+ CD25+ regulatory T cells [24–27], reversing immunologic skew (favoring T helper type I immunity over T helper type 2 immunity) [23] and promoting the evolution of a memory T cell response [28]. DOX augments CD8+ T cell responses when given after chemotherapy in some systems [22,23] and prior to chemotherapy in others [29]. PTX and DTX are also key components of systemic therapy for breast cancer, and both have immunomodulatory

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CY − DOX + CD4+CD25+ Regulatory T Cells High Avidity CD8+ T Cell PTX +

Immature DC

Low Avidity CD8+ T Cell

Mature DC

CY, PTX

CD4+ T Helper Type 1 Cell

Figure 11.1. Chemotherapy modulates immune tolerance. Standard breast cancer therapeutics can modulate immune tolerance. Paclitaxel (PTX) can facilitate dendritic cell (DC) maturation by binding to toll-like receptors. Both PTX and cyclophosphamide (CY) promote the differentiation of T helper type 1 cells relative to T helper type 2 cells. CY can promote the recruitment of high-avidity CD8+ T cells to the immune response by abrogating the suppressive influence of CD4+ CD25+ regulatory T cells; the impact of this influence on recruitment and activation of low-avidity CD8+ T cells remains unclear. Doxorubicin (DOX) can augment the activity of effector CD8+ T cells; the influence of DOX on T cell avidity remains unclear.

potential beyond the ability to induce apoptosis [30]. PTX mimics the activity of lipopolysaccharide (LPS) by binding to the toll-like receptor 4 (TLR4) expressed by murine dendritic cells (DCs) [31,32]; in humans these two drugs have a similar activity that is dependent on Myd88 and likely occurs via a distinct TLR [33]. Like CY, PTX also favors the evolution of T helper type 1 responses; this immunodulatory activity of PTX occurs only at low doses, and only when the drug is given prior to antigen exposure [23]. The time dependence of the idiosyncratic adjuvant activity of these drugs is clear, and suggests synergy at the time of immune priming for CY (CD4+ CD25+ regulatory T cells) and PTX (DC modulation/tumor cell apoptosis), and at the time of effector activity for PTX and DOX (tumor cell apoptosis, and increasing with direct priming, cross-priming, or both).

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Immunosuppressive Pathways in the Tumor Microenvironment Chemotherapy can also modulate the tumor microenvironment, rendering it more receptive to a productive antitumor immune response. CY, PTX, DOX, and vinblastine given at low doses frequently (metronomic chemotherapy) preferentially target the tumor vasculature over transformed tumor cells [34]. This may increase both tumor cell apoptosis and vascular access early in treatment. Chemotherapy with drugs like 5 -aza-2 -deoxycytidine and 5-fluoruracil can upregulate the expression of molecules involved in antigen processing and presentation, thereby rendering tumor cells more sensitive to lysis by cytotoxic T lymphocytes (CTL) [35,36]. This has been demonstrated in tumor cell lines in vitro. Other chemotherapeutics may upregulate costimulatory molecules within the tumor microenvironment, thereby enhancing T cell activity [35–39].

11.4 MONOCLONAL ANTIBODY AND ENDOCRINE THERAPY FOR BREAST CANCER AND TUMOR IMMUNITY Monoclonal Antibody Therapy Therapeutic monoclonal antibodies that are specific for the tumor cell are promising for combination immunotherapy because they provide a strategy for passively restoring the antigen-specific humoral immune response in the context of activating the cellular immune response with cancer vaccines. Trastuzumab is a humanized monoclonal antibody specific for HER2/neu [2], a protooncogene overexpressed by up to 25% of human breast cancers [40,41]. HER2/neu-overexpressing tumors are more aggressive, relapse earlier, and are associated with a higher mortality than the majority of breast cancers. Trastuzumab is now in widespread use for the treatment of breast cancer at every stage except ductal carcinoma in situ. This antibody has the capability of potentiating tumor immunity in several ways: 1. Trastuzumab interferes with signaling pathways that promote tumor cell growth and metastasis [42]. 2. Like standard-dose chemotherapy, trastuzumab induces significant tumor cell apoptosis in human breast tumors even after only one dose [43]. 3. Trastuzumab induces the ubiquitination and degradation of internalized HER2/ neu molecules [44,45], thus increasing proteasome-dependent antigen presentation and augmenting the lytic activity of MHC class I–restricted CTLs against HER2/neu-positive target cells [46,47]. 4. Trastuzumab recruits innate immune effectors to the tumor microenvironment to facilitate antibody-dependent cellular cytotoxicity (ADCC) [48,49].

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The pleiotropic nature of these immunomodulatory activities is striking, as trastuzumab in combination with tumor vaccine has the potential for augmenting innate, humoral, and cellular immunity in a highly concerted fashion. Bevacizumab is a humanized monoclonal antibody specific for the vascular endothelial growth factor (VEGF), a cytokine that both promotes the growth of the tumor-associated vasculature and suppresses tumor-associated immune responses. Although not yet FDA-approved for the management of breast cancer, phase III clinical trial data are compelling enough to warrant the widespread use of bevacizumab in combination with chemotherapy in the management of metastatic breast cancer. The reported data support its use as first-line therapy for metastatic breast cancer in combination with standard-dose PTX. VEGF itself can cause thymic atrophy, inhibit T cell development, and diminish DC function [50–52]. Importantly, monoclonal antibodies specific for VEGF can increase the number and function of DCs in tumor-bearing mice, thereby facilitating the effectiveness of DC-based immunotherapy [53]. Together, these observations suggest that VEGF blockade may augment the activity of tumor vaccines, or the function of adoptively transferred tumor-specific lymphocytes. Cetuximab is a humanized monoclonal antibody specific for the epidermal growth factor receptor (EGFR), a transmembrane tyrosine kinase receptor overexpressed by a variety of epithelial tumors, including colorectal, aerodigestive, prostate, ovarian, and breast cancers. Notably, the EGFR protein has emerged as a prominent molecular feature of the basal-like subset of breast cancers. This breast cancer subset is “triple negative”; that is, it does not express the estrogen receptor (ER), the progesterone receptor (PR), or HER2/neu. Therefore, no targeted therapies exist to complement chemotherapy for the treatment of this breast cancer subtype, and it poses an unmet medical need. Like trastuzumab, cetuximab inhibits the signaling pathways that promote tumor growth and progression, and also induces tumor apoptosis. In particular, cetuximab can synergize with PTX to reduce angiogenesis and induce tumor cell apoptosis [54]. Immunomodulatory activity for cetuximab has not been reported. Endocrine Therapy There is a well-defined subset of breast cancers with a high likelihood of response to endocrine therapy. Frequently, these tumors are much more responsive to manipulating the hormonal milieu than they are to systemic chemotherapy. Endocrine therapy for breast cancer includes the selective estrogen receptor modulators and destroyers (SERMS and SERDS: tamoxifen and raloxifene, and fulvestrant), the aromatase inhibitors (anastrozole, letrozole, and exemestane), and ovarian ablation. Despite a clear role for estrogen in the growth and development of lymphocytes, there is little information available about the potential impact of these therapies on tumor immunity. Tamoxifen and raloxifene can inhibit the differentiation and LPS-induced maturation of DCs, antagonizing the ER and maintaining the DCs in an immature state in vitro [55,56]. Aromatase inhibitors can sensitize tumor cells to monocyte-mediated ADCC [57]. While no immunomodulatory effects of

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ovarian ablation have been reported, it is interesting that androgen ablation in murine models of prostate cancer can diminish immune tolerance [58]. These findings warrant further investigation of these interactions in clinically relevant breast cancer models. 11.5


A Stringent Model for Human Breast Cancer Immunotherapy The neu transgenic mouse model is a highly selective, clinically relevant laboratory model for discriminating the most active multimodality immunotherapeutic regimens for testing in breast cancer patients. Studies have shown that neu mice stochastically develop mammary carcinomas that develop in a histologic sequence that recapitulates the progression of human breast cancers [59]. They develop spontaneous mammary carcinomas that overexpress the protooncogene HER2/neu, and represent an informative model system for the biology of HER2/neu-positive breast cancers. Moreover, they display a profound immune tolerance to the HER2/neu oncoprotein [60] and develop very low levels of HER2/neu-specific antibodies and T cells that fail to prevent tumor progression, thereby recapitulating the immunobiology of breast cancer patients [60–62]. Granulocyte–macrophage colony-stimulating factor (GM-CSF)-secreting cellbased vaccination is an active vaccination platform for cancer immunotherapy [63]. This immunotherapeutic strategy capitalizes on the ability of GM-CSF to recruit and activate DCs, widely regarded as the most effective professional antigen-presenting cell (APC) [64,65]. The human vaccine is typically composed of one or more tumor cells that are genetically modified to secrete GM-CSF, and is inoculated intradermally [66,67]. There, the paracrine secretion of GM-CSF recruits and activates host DCs, which take up and process tumor antigens shed by the vaccinating tumor cells for presentation to the immune system. This approach has two major advantages: (1) the cross-priming mechanism of immune activation obviates the need to match the vaccine cells to the MHC haplotype of the host, and (2) the use of cells to deliver pertinent tumor antigens allows the delivery of multiple tumor antigens in an unbiased way, activating a multitargeted immune response to antigens that are both known and unknown. Therefore, the vaccination strategy utilizes an optimal mechanism that maximizes the specificity and diversity of immune priming. GM-CSF-Secreting Vaccines and neu Transgenic Mice The Jaffee group has interrogated the immunobiology of neu transgenic mice using GM-CSF-secreting vaccines. Neu transgenic mice were derived from FVB/N mice by introducing the rat neu protooncogene into the germline DNA under the control of the mammary-specific promoter MMTV [59]. They spontaneously develop HER2/neu-positive breast cancers that can metastasize to the lung, and less commonly to other sites. Vaccinating tumor-free neu mice with a GM-CSF-secreting,

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HER2/neu-specific cell-based vaccine can delay the outgrowth of both spontaneous and transplanted HER2/neu-positive tumors [60]. In contrast, vaccinating tumor-bearing neu transgenic mice with the same vaccine has absolutely no impact on tumor growth [23]. These observations highlight the impact of tumor burden on the efficacy of cancer vaccines in the setting of immune tolerance. Comparing the immune response of parental FVB/N mice and neu transgenic mice to HER2/neu-targeted vaccination highlights the immense impact of immune tolerance itself on vaccine efficacy. Vaccinating tumor-bearing (>0.5 cm), nontolerant, parental FVB/N mice produces a complete and durable tumor rejection. This tumor rejection response is associated with an immune response characterized by robust HER2/neu-specific antibody titers, and significant numbers of active, HER2/neu-specific CTLs [23,60]. These CTLs are almost exclusively specific for the immunodominant epitope of rat HER2/neu, RNEU420 – 429 (p50) [68]. In contrast, vaccinating tumor-bearing (nonpalpable), tolerant neu transgenic mice is completely ineffective in delaying tumor outgrowth [23]. The immune response induced by vaccination is tepid, with very low titers of HER2/neu-specific antibodies, and low numbers of HER2/neu-specific T cells. Notably, the T cell population that does develop contains a paucity of T cells specific for the immunodominant epitope p50 [23,25]. This immune response is so weak that the tumor outgrowth curves of mice vaccinated with cells that express HER2/neu and secrete GM-CSF overlap completely with those of mice vaccinated with cells that are HER2/neu-negative and secrete only GM-CSF. By combining vaccination with established breast cancer therapeutics, it is possible to partially overcome this immune tolerance, enhancing vaccine activity in tolerant neu mice. Combining GM-CSF-Secreting Vaccination and Chemotherapy in neu Transgenic Mice The Jaffee group has conducted a systematic analysis of chemotherapy in sequence with HER2/neu-directed, GM-CSF-secreting vaccination in neu transgenic mice [23]. In this study, low doses of CY and PTX were found to augment vaccine activity if given prior to vaccination, but not if given after vaccination. In contrast, low doses of DOX were found to augment vaccine activity if given after vaccination, but inhibited vaccine activity when given prior to immunization. Notably, combining both CY and DOX with vaccination produced the greatest antitumor effect, curing up to 30% of neu transgenic mice of preexisting tumors. Studies to elucidate the mechanism by which chemotherapy augments vaccine activity revealed several important findings: 1. The CD4+ T helper type I response is facilitated by both CY and PTX pretreatment; giving these drugs after vaccination actually inhibits this response [23]. This observation supports the reports that CY and PTX can reverse the immunologic skew that antagonizes effective tumor immunity in the setting of established tumor burdens.


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2. CY abrogates the immunosuppressive impact of cycling CD4+ CD25+ regulatory T cells (Tregs) on the induction of effective CD8+ CTL in tumor-bearing, tolerized neu transgenic mice [25]. Importantly, latent, high-avidity p50specific T cells are recruited to the immune response of cured tumor-bearing neu transgenic mice when the influence of these cycling Tregs is alleviated. This is an important observation, because it demonstrates that high-avidity, tumor-specific T cells can exist in the periphery, and can be recruited to the antitumor response with the appropriate vaccination regimen. Moreover, it is consistent with observations by other groups that CY can mitigate the influence of Tregs. Another group has shown that CY can induce type I interferons, thereby promoting the evolution of a CD44hi memory T cell response [28]. 3. Our preliminary data suggests that low-dose PTX administered prior to GMCSF-secreting vaccination in neu transgenic mice augments tumor immunity by signaling through TLR4 at the time of immune priming (Jaffee EM, unpublished data). The immunomodulatory activity of DOX in neu transgenic mice has not been evaluated, but the drug promotes the evolution of tumor-specific CD8+ T cells when given after GM-CSF-secreting vaccination in the nontolerant CT26 mouse model of colon cancer [22].

GM-CSF-Secreting Vaccination and Monoclonal Antibody Therapy in neu Transgenic Mice Our group has also shown that the combination of HER2/neu-specific monoclonal antibodies and vaccination are more effective than either alone in neu transgenic mice [45]. In vitro studies have demonstrated that HER2/neu-specific monoclonal antibodies can enhance the lytic activity of MHC class I–restricted HER2/neu-specific CTLs against HER2/neu-positive targets [46,47]. Consistent with this concept, the passive administration of both HER2/neu-specific antibodies and HER2/neu-specific CTLs induces a more effective antitumor response than either alone in severe combined immmunodeficient mice [69]. Building on this observation, the passive administration of HER2/neu-specific monoclonal antibodies in combination with vaccination in neu transgenic mice has a greater antitumor effect than does either alone, with combined therapy curing about 40% of tumor-bearing mice [45]. Combination therapy augments the number of vaccineinduced, tumor necrosis factor (TNF)-secreting, HER2/neu-specific CTLs as measured by ELISPOT. Notably, the monoclonal antibodies alone induce new CD8+ T cell immunity specific for HER2/neu. These observations are consistent with observations that humoral HER2/neu-specific immunity induced after peptide vaccination in patients with breast cancer can inhibit tumor cell growth and signaling [70]. We have more recently shown that mitigating the influence of Tregs with low-dose CY in the setting of vaccination and weekly therapy with HER2/neu-specific monoclonal antibodies is the most effective, curing almost 60% of neu transgenic mice of preexisting tumors (Emens LA, unpublished data). This is associated with both increased tumor cell apoptosis as determined by immunohistochemical staining of

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tumors for cleaved caspase 3, and by both increased numbers of HER2/neu-specific CD8+ T cells and higher HER2/neu-specific antibody titers. Clinical Trials of GM-CSF-Secreting Vaccination with Standard Breast Cancer Therapeutics We have developed a human allogeneic, GM-CSF-secreting breast tumor vaccine for clinical administration [67]. The vaccine itself is composed of two cell lines, SKBR3 and T47D, both of which have been genetically modified to secrete human GM-CSF by plasmid DNA transfection. The vaccine is designed to account for the majority of breast cancers that are seen clinically, with T47D representing ER-positive and HER2/neu-negative breast cancers, and SKBR3 representing ER-negative and HER2/neu-positive breast cancers. We are currently accruing to two clinical trials that are testing a fixed dose of 5 × 108 cells of this GM-CSF-secreting breast tumor vaccine in sequence with standard breast cancer therapeutics in patients with metastatic breast cancer. The first study tests this fixed dose of vaccine in sequence with a range of doses of CY (250–450 mg/m2 ) and DOX (15–35 mg/m2 ) given 1 day prior to and 7 days after vaccination, respectively, in patients with stable metastatic breast cancer [71]. The goal of this study is to assess the bioactivity of the vaccine by measuring immune responses to HER2/neu (delivered by the SKBR-3 component of the vaccine), and to determine which doses of sequential chemotherapy produce the greatest immune response. Thus far, the vaccine has been quite safe, with side effects largely limited to local injection site reactions consisting of erythema, induration, and pruritis. Analysis of HER2/neu-specific immune responses is just beginning. We have just opened a second study that tests this same fixed dose of 5 × 108 vaccine cells with CY modulation (300 mg/m2 ) in the setting of weekly trastuzumab therapy in patients with HER2/neu-overexpressing metastatic breast cancer. This second study is distinguished from the first in that it aims to augment vaccine activity by (1) enhancing antigen processing and presentation (trastuzumab) and (2) mitigating the influence of Tregs (CY). Importantly, both of these studies allow the concomitant administration of endocrine and bisphosphonate therapy as appropriate.

11.6 THE FUTURE: MANIPULATING IMMUNOLOGIC CHECKPOINTS WITH NOVEL IMMUNE-BASED THERAPIES It is clear that standard breast cancer therapeutics can exert an immunomodulatory effect that can be harnessed for immunotherapeutic benefit. The next generation of breast cancer vaccine studies will surely combine breast tumor vaccines with agents that manipulate immunologic checkpoints. These checkpoints represent critical control loci for upregulating or downregulating the antitumor immune response

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[72,73]. During immune priming, the strength of T cell activation is determined by the primary signal conferred by the recognition of the antigen–MHC complex by the T cell receptor, and the secondary signals contributed by the summation of positive and negative accessory signals for T cell activation. It is possible that the antitumor T cell response can be facilitated by amplifying positive costimulatory signals for T cell activation, or by blocking negative accessory signals for T cell activation. For breast cancer, the positive regulatory pathway with the most promise to date is the OX40 pathway [74]. OX40 signaling supports the activation, expansion, and survival of both CD4+ and CD8+ T cells, and can abrogate immune tolerance in vivo. The treatment of tumor-bearing mice with monoclonal antibodies specific for OX40 alone resulted in prolonged CD4+ and CD8+ T cell-dependent tumor-free survival [74]. Furthermore, we have shown that HER2/neu-targeted vaccination combined with OX40 monoclonal antibody therapy in tumor-bearing neu transgenic mice could overcome immune tolerance, augmenting tumor-free survival and the HER2/neu-specific T cell response in about 20% of neu transgenic mice [75]. Importantly, analysis of vaccine-induced immune responses specific for the immunodominant epitope showed that vaccination alone induced a small and transient increase of p50-specific T cells that were largely ineffective in delaying tumor growth, whereas the addition of therapy with a monoclonal antibody specific for OX40 resulted in the sustained induction of long-lived p50-specific T cells of high avidity. Single-agent monoclonal antibody therapy targeting OX40 is just now entering human clinical trials. The negative regulatory pathway with the most promise to date is the CTLA4 counterregulatory signaling pathway [76]. CTLA4 signaling serves as a check on the process of T cell priming, both raising the threshold for T cell activation and limiting T cell expansion. Notably, CTLA4 is also expressed at significant levels by Tregs, and modulating this pathway can affect the primary T cell response as well as the complementary Treg response. Although little data exist for breast cancer, several clinical trials have tested monoclonal antibodies specific for CTLA4 as a single agent, either subsequent to or in active sequence with vaccination in other tumor types [77–80]. In the aggregate, these clinical trials have been quite promising, with evidence of increased tumor-specific immune responses and tumor regression. Notably, bioactivity has tracked with symptoms of significant autoimmunity, further supporting the potency of this drug as an immunomodulator with promise. Currently (as of 2007), CTLA4-specific monoclonal antibody therapy is being tested in combination with endocrine therapy in patients with metastatic breast cancer. A multitude of immune checkpoint modulators might be tested for activity in combination with breast cancer vaccines in the future. Those that are currently available for clinical use include monoclonal antibody specific for CD25 (daclizumab) and denileukin diftitox (Ontak), both of which would be used to abrogate the negative influence of regulatory T cells. These two drugs target the interleukin 2 (IL-2) receptor, which is overexpressed on Tregs. Supporting its use, denileukin

180


diftitox has been demonstrated to deplete Tregs, resulting in a marked reduction of tumor growth in neu transgenic mice [81]. It has also been shown to deplete Tregs in humans [82,83]. Treg modulators that might be of future use include those that target the glucocorticoid-induced tumor necrosis factor receptor (GITR) and lymphocyte-associated globulin 3 (LAG3) [83]. Other promising agents include monoclonal antibodies or other drugs specific for the B7 family of positive and negative accessory molecules of T cell activation, agents specific for the CD40 pathway, and drugs that target the 4-1BB pathway [72].

11.7

SUMMARY AND CONCLUSIONS

Improvements in the management of breast cancer have long led revolutionary shifts in cancer care. Targeted therapy, exemplified by the use of tamoxifen to treat breast cancers that overexpress the estrogen receptor α (ERα), has been a mainstay of breast cancer therapy since the mid-1970s. Increasing numbers of molecularly targeted drugs are entering the treatment armamentarium for breast cancer as well as other cancer types. More recently, insights elucidated by the genomic revolution have opened the door for individualized therapy based on fundamental tumor biology rather than anatomic staging. Breast cancer genomics also represents a framework for the application of new targeted therapies designed to disrupt the key regulatory pathways that underlie the biology of distinct breast tumor types. However, continued improvements will likely continue to be limited by the specter of fixed drug resistance, arguing for innovative treatment strategies that work by a completely different mechanism. Tumor vaccines represent a unique therapeutic strategy that aims to retool the host–tumor interaction to facilitate tumor rejection. Importantly, multiple established and novel breast cancer therapies have immunomodulatory potential. Breast cancer thus represents a powerful model system for elucidating mechanisms of therapeutic synergy between breast tumor vaccines and both standard and novel breast cancer therapeutics. Capitalizing on this opportunity will ensure that breast cancer morbidity and mortality continue to decline. Breast cancer vaccines have great promise for further improving clinical outcomes in the management of breast cancer. They function by a mechanism completely different from that of any breast cancer therapy to date, and have potential for breast cancer prevention as well as treatment. It is clear that their clinical development will be accelerated and optimized by taking into account how the immune system interacts with the disease, and how the vaccine-induced immune response interacts with established breast cancer therapeutics. The clinical success of trastuzumab illustrates the potential of immune-based therapy for this disease. Close collaboration between laboratory and clinical researchers will ensure that immune-based therapy continues to impact breast cancer outcomes in profound ways.

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Conflict of Interest This chapter describes work using granulocyte–macrophage colony-stimulating factor-secreting tumor vaccines. Under a licensing agreement between Cell Genesys, Incorporated and the Johns Hopkins University, the university is entitled to a share of royalty on sales of products described in this chapter. The terms of this agreement are being managed by the Johns Hopkins University in accordance with its conflict-of-interest policies.

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12 T-REGULATORY CELL MANIPULATION IN TUMOR IMMUNOTHERAPY ¨ Jens Ruter, Brian G. Barnett, Ilona Kryczek, Michael J. Brumlik, Benjamin J. Daniel, George Coukos, Weiping Zou, and Tyler J. Curiel

12.1

INTRODUCTION

Established malignant tumors express a variety of tumor-associated antigens (TAA), yet rarely elicit an endogenous immune response that effectively eradicates tumor. Much evidence suggests that specific immune surveillance can eliminate tumors at early stages, whereas established tumors generally induce immune tolerance [1]. It has now been demonstrated that numerous tumor-derived factors contribute to immune tolerance and immunosuppression in the tumor microenvironment [2], helping explain the relative lack of effective immune surveillance at later stages of tumor development. Earlier concepts that simply boosting the numbers or function of effector cells alone would be therapeutically useful have given way to the sobering realization that active tumor-mediated mechanisms can defeat antitumor effector functions. Thus, effective cancer vaccines must overcome the immunosuppressive tumor microenvironment to help stimulate a useful antitumor immune response. Cancer Vaccines and Tumor Immunity Edited by Rimas Orentas, James W. Hodge, and Bryon D. Johnson Copyright © 2008 John Wiley & Sons, Inc. 189

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Engendering strong antitumor immunity may involve breaking Treg-mediated peripheral tolerance to tumor-associated antigens (TAAs). Tregs are emerging as important mediators of tumor microenvironmental immunosuppression in patients with cancer, including suppression of TAA-specific immunity. Consistent with this concept, experimental depletion of Tregs in tumor-bearing mice improves endogenous immune-mediated tumor clearance [3], improves TAA-specific immunity [4], and enhances the efficacy of tumor immunotherapy such as vaccination [5] or CTLA4 blockade [6]. Nonetheless, as Tregs also control self-reactive T cells present in all individuals, including those mediating homeostatic peripheral tolerance, the therapeutic benefit of attempting to deplete TAA-reactive Tregs may be limited by the development of pathologic autoimmune phenomena if homeostatic Tregs maintaining normal peripheral tolerance are also depleted. Further, more recent data suggest that cancer vaccines may also expand existing pools of tumor microenvironmental Tregs [7], which has potential to exacerbate immunosuppression further. Certain subsets of dendritic cells can also expand various Treg subsets [8,9], which may also contribute to this problem. Although CD4+ Tregs have been studied and characterized extensively (reviewed in Refs. 10 and 11), many questions regarding their origins and functions remain. The best known express the CD4+ CD25+ Foxp3+ phenotype and are referred to as regulatory T lymphocytes, T-regulatory cells, or Tregs [12–14], as we refer to them here. Tregs can suppress the function of tumor-specific CD4+ [15] or CD8+ [16] T effector cells by mechanisms that are not fully understood, but most likely include soluble factors and cell-to-cell contact [10,11,16–18]. Functional Tregs circulate in increased numbers in peripheral blood of patients afflicted with many types of human cancers [3,10,11,19–26] and in the solid tumor mass [20]. Human CD4+ CD25+ Tregs inhibit TAA-specific immunity, allow tumor growth in the presence of TAA-specific immunity and predict poor survival in human ovarian cancer [20]. Foxp3 expression also correlated with poor survival in ovarian cancer [27]. Thus, reducing tumor microenvironmental Treg function is emerging as a means to improve the efficacy of tumor immunotherapy. We have defined four general strategies to reduce Treg function for potential use in immunotherapy. The first strategy, killing of Tregs, which can be effected using monoclonal antibodies, targeted toxins, selected chemotherapeutic agents, or other molecules. Alternative strategies include blockade of Treg function, trafficking, or differentiation. Some of these strategies have already been tested clinically, while others are under preclinical investigation. Each strategy is discussed below as it relates to CD4+ CD25+ Tregs, although such strategies could also be applied to other types of regulatory cells. 12.2

T-REGULATORY CELL DEPLETION

Monoclonal Antibodies against CD25 CD25, the α-chain of the IL-2 receptor, is constitutively expressed on many tumor-associated Tregs and is thus a candidate molecule to target them for depletion. Injection of the anti-CD25 monoclonal antibody PC61 mediated an apparently

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rapid and efficient depletion of CD4+ CD25+ Treg function [4] and promoted tumor injection in mice [3]. However, more recent results suggest that the mechanism of action of PC61 monoclonal antibody treatment is functional inactivation, not depletion of Tregs [28]. Nonetheless, the net effect is reduced Treg activity with concomitantly improved antitumor immunity. Two antihuman CD25 antibodies, daclizumab (Zenapax) and basiliximab (Simulect), are approved for human use in transplantation, autoimmunity, and cancers including HTLV1-induced adult T cell lymphoma/leukemia (reviewed in Ref. 29). There are no published reports to our knowledge on the efficacy of these anti-CD25 antibodies to deplete Tregs in humans, but anecdotal information suggests only very limited, if any, such activity. A definitive report describing the effects of these antibodies on tumor-related Tregs would be very useful. Ontak Ontak, DAB389 IL-2 (denileukin diftitox), is a recombinant diphtheria toxin conjugate consisting of the active domain of diphtheria toxin and human interleukin 2 (IL-2). It is approved by the FDA for the treatment of cutaneous T cell leukemia/lymphoma [30]. It targets cells expressing the IL-2 receptor [31,32] and is proposed to be internalized into CD25+ cells by endocytosis, where it inhibits protein synthesis, leading to apoptosis. More recent evidence demonstrates that Ontak also targets cells through components of the IL-2 receptor in addition to CD25 [32,33] (also Curiel and coworkers, unpublished results). Because of the phenotypic similarity between CD4+ CD25+ cutaneous T cell leukemia/lymphoma cells, for which Ontak is approved for treatment, and CD4+ CD25+ Tregs, we hypothesized that Ontak would deplete CD4+ CD25+ Tregs in humans. We undertook a clinical trial demonstrating that in blood of human patients with cancer, Ontak depleted functional Tregs, correlating with improved measures of immunity [34] and therapeutic benefit [35]. A clinical trial [36] showed that pretreatment with Ontak improves the immunogenicity of vaccination with RNA-transfected dendritic cells, enhancing the stimulation of tumor-specific T cells in patients with renal cell cancer compared to vaccination alone. This report also demonstrated that Ontak depleted functional Tregs in blood, and the authors speculated that as the mechanism of action. Ontak depletes tumor-associated Tregs and improves immune-mediated rejection of tumor in a mouse model of breast cancer [37]. Nonetheless, a clear link between Treg depletion and any immunologic finding or clinical improvements in any reports in humans to date remains to be determined. In another clinical study [38], treatment with Ontak did not induce any objective response in human melanoma or renal cell carcinoma and evinced no reduced suppressive effect of CD4+ CD25+ cells in vitro. Why Ontak did not reduce Treg function in this study is unknown, but could relate to the dose or schedule of Ontak used, differences in the immunobiology of different cancers, prior treatment of patients with IL-2, or other factors. For example, IL-2 can increase numbers of CD4+ CD25+ Foxp3+ Tregs in blood of patients with renal cell carcinoma or

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melanoma [39] and pediatric sarcoma [40]. Furthermore, IL-2 signaling through the IL-2 receptor on Tregs increases their survival and activates their suppressor function [41]. As recently activated T cells also express high-level CD25, Ontak could also deplete CD25+ effector cells. We observed effector cell depletion in some of our patients following three or more weekly Ontak infusions at 12 μg/kg (Curiel and coworkers, unpublished results), which has also been suggested by in vitro work [36]. An optimal balance between effector and Treg depletion may be achieved using different Ontak doses or schedules. We are currently testing monthly Ontak infusion in this regard. Cytotoxic Chemotherapy Cyclophosphamide Cyclophosphamide is an alkylating agent widely used in various chemotherapeutic regimens. Low-dose cyclophosphamide can reduce the number of Tregs and thus may be beneficial as an immune-modulating agent, even in the setting of cyclophosphamide-resistant tumors [42,43], although the mechanism is not well understood [44]. Depletion of Tregs by cyclophosphamide in a rat tumor model potentiates the efficacy of an active tumor-specific immunotherapy [45]. Treg depletion by low-dose cyclophosphamide in humans has not yet been convincingly demonstrated [46]. Further investigations of cyclophosphamide actions apart from its direct tumor cytotoxicity are warranted, especially since low-dose cyclophosphamide has also been shown to be antiangiogenic [47]. Fludarabine Fludarabine is a purine analog used to treat chronic lymphocytic leukemia [48]. It has been to reduce or abolish the suppressive activity in the CD4+ CD25+ T cell population of patients with chronic lymphocytic leukemia [49]. As Tregs may play a role in the dysfunctional immune system of patients with chronic lymphocytic leukemia, the effect of fludarabine on Tregs in this and possibly other malignancies merits further investigation. 12.3

FUNCTIONAL BLOCKADE OF T-REGULATORY CELLS

Anti-GITR Antibodies The GITR (glucocorticoid-induced tumor necrosis factor receptor-related gene) is a cosignaling antigen expressed on murine [50] and human [51] T lymphocytes. This antigen is expressed constitutively at high levels on Tregs and is further upregulated on T cell activation [52]. Stimulation of GITR by monoclonal antibodies in vitro abrogates the suppressor function of murine Tregs [52,53], but a similar

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effect has not been demonstrated convincingly in human Tregs [54]. Treatment of tumor-bearing mice with an agonistic anti-GITR monoclonal antibody provoked potent tumor-specific immunity and eradicated established tumors without eliciting overt pathologic autoimmunity [55]. The molecular mechanisms of this effect were not characterized in detail but likely involve attenuation of Treg-mediated suppression in the presence of antigenic stimulation [55]. This strategy has not yet been tested in a human clinical trial to our knowledge. TLR Signaling Approximately 15 toll-like receptors (TLRs) are ubiquitously expressed on various mammalian cells, including human Tregs and dendritic cells [56]. They bridge the innate and the acquired immune system by recognizing a set of bacterial and viral pathogen-associated molecular patterns, and initiate important steps in generating a specific immune response, such as dendritic cell maturation [56]. The suppressive function of Tregs can be directly abrogated in vitro by TLR-derived signals [57]. The importance of TLR signaling for tumor immunotherapy is demonstrated by in vitro experiments showing that only virus-based vaccines provide TLR signals required for reversing Treg-mediated tolerance. Conversely, dendritic cell (DC)-based vaccines lack TLR signaling, and can break tolerance of CD8+ cells only after removal of Tregs or with the coadministration of another TLR ligand [58]. These experiments suggest that functional Treg inhibition by TLR ligation (such as with TLR9 ligation by CpG oligonucleotides) may be a novel means to boost efficacy of vaccine-based cancer treatments. Combining dendritic cell-based vaccines with Treg depletion is another alternative strategy, as discussed above. Anti-CTLA4 Antibodies Cytotoxic T lymphocyte antigen 4 (CTLA4) is strongly expressed on Tregs [10,11,59,60] as well as on certain other CD4+ and CD8+ T lymphocytes [61]. As this molecule downregulates T cell responsiveness [62,63], CTLA4-mediated inhibition may restrict T cell activation during the initiation and progression of the antitumor immune response [61]. CTLA4 blockade by monoclonal antibodies elicits a strong antitumor response in a murine melanoma model [6]. A human anti-CTLA4 monoclonal antibody has been tested in various different cancers in phase I clinical trials [64] with promising results, although pathologic autoimmune phenomena are significant side effects. CTLA4 is also expressed on effector T cells. An analysis of the cellular mechanisms of CTLA4 blockade suggests that the antitumor effects of CTLA4 blockade are due to a direct effect on CD4+ and CD8+ effector T cells, and not to an inhibition or depletion of Tregs [65]. The significant symptoms of pathologic autoimmunity observed with anti-CTLA4 treatment [65] may possibly be controlled by using a different antibody clone. Thus, whereas CTLA4 blockade is a promising candidate for immunotherapy, it may not significantly manipulate Treg function. On the other hand, raising the threshold of effector cells to Treg suppression (such as with anti-CTLA-4 antibody) is an additional strategy to combat immunopathology.

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Treg Trafficking Blockade Chemokine Signaling Chemokines are a family of chemotactic molecules that modulate trafficking of immune cells through generation of a concentration gradient between normal and pathologic tissues [66]. While various normal tissue types produce chemokines, cancers also secrete them [67]. We have identified the tumor-secreted chemokine CCL22 as a candidate mechanism for mediating Treg migration into the microenvironment of human ovarian cancer [20]. We also found that CCR4 (a receptor for CCL22) is expressed on the majority of Tregs in ovarian cancer. In vivo blockade of CCL22 with a monoclonal antibody significantly reduced Treg migration into ovarian tumors in an immunodeficient murine xenograft model, improving immune rejection [20]. Thus, blocking Treg trafficking is a potential strategy for treating human cancers. However, CCL22 signals may also regulate the trafficking of effector T cells. Therefore, the net benefit of CCL22 blockade, or effects of interrupting other trafficking signals in immunocompetent hosts requires further investigation as to potential therapeutic utility. Ovarian cancer cells also produce large quantities of the chemokine CXCL12 [also known as stroma-derived factor 1 (SDF1)], which can attract dysfunctional plasmacytoid dendritic cells [75], which in turn can induce differentiation of CD8+ regulatory T cells [68]. Thus, blocking CXCR4 signals may be therapeutically useful to reduce Treg numbers or function in ovarian cancer, in addition to prior proposals to use it to block metastasis. More recent data suggest that IL-2 promotes Treg cell migration into tumors by augmenting their CXCR4 expression, a receptor for CXCL12 [69], suggesting an additional level of regulation that may be exploitable therapeutically. Various chemokines and chemokine receptors have been targeted successfully in animal models of arthritis [70], and selective small-molecule chemokine receptor antagonists have entered phase I clinical trials [71], including CXCR4 signaling antagonists. However, therapeutic success of chemokine or receptor antagonism may be limited by the binding diversity of chemokine receptors for multiple ligands, the redundancy of the individual chemokine–ligand pair, and the role played by chemokines in normal tissue homeostasis or antitumor defense. These additional effects of chemokine–receptor antagonism must be considered in chemokine–receptor blocking strategies.

12.4

INTERACTIONS BETWEEN T-REGULATORY AND OTHER CELLS

Interactions between Dendritic Cells and Treg Cells Dendritic cells are a heterogeneous group of antigen-presenting cells with a variety of differences in anatomic localization, cell surface phenotype, and immunologic function [72]. Human dendritic cells are typically divided into two main populations: myeloid (MDC) and plasmacytoid (PDC) [72]. While it is well established

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that dendritic cells are responsible for initiating or upregulating immune responses, it has also been shown more recently that they are also capable of inducing immune tolerance [2]. We have identified several mechanisms whereby dendritic cells may induce Tregs that contribute to impaired tumor immunity [2,8,9,73]. Thus, it may be possible to modulate the suppressive activity of Tregs by blocking dendritic cell trafficking or function in the tumor environment and thereby subsequently enhance tumor immunity as discussed further below. Interactions between PDCs and T Cells Plasmacytoid dendritic cells are a subtype of dendritic cells responsible for producing type I interferons in peripheral blood in response to viruses and other stimuli [74]. In contrast to normal blood PDCs, tumor environmental PDCs promote tumor vascularization [75], and induce differentiation of IL-10-expressing T cells [9]. Tumor PDC-activated T cells include CD8+ CCR7+ CD62L+ IL-10+ T cells, which suppress effector T cell function through IL-10 production [9] and are functional CD8+ Tregs. Thus, interactions between PDCs and CD8+ T cells may contribute to the differentiation of CD8+ Tregs in the tumor microenvironment. This differentiation could potentially be inhibited by blocking tumor PDC–T cell interactions. The chemokine CXCL12 induces ingress of PDCs into the tumor environment through CXCR4 signals in ovarian cancer [73]. We showed that blocking CXCR4 with specific antibodies reverses tumor-mediated protection of PDCs from apoptosis, chemotaxis, and adhesion/transmigration in vitro [73]. Various bicyclam compounds with specific antagonistic activity for CXCR4 (AMD3100 [76] or AMD3465 [77]) are effective in various clinical settings such as stem cell mobilization and human immunodeficiency virus infection [78]. They may also be useful in the local tumor environment by blocking PDC ingress, thereby preventing PDC–T cell interactions (in addition to blocking tumor metastasis through CXCR4 signals, or other pathologic activities). An alternative approach is to deplete PDCs selectively. Relatively specific surface antigens on human PDCs (such as BDCA2 or BDCA4) [79] have been described. However, a PDC-depleting antibody has not yet been tested in humans. Thus, while the manipulation of PDC–T cell interactions in the context of tumor immunity represents an attractive target for tumor immunotherapy, more research must be done to establish its clinical efficacy and a suitable means to effect it. Interactions between MDCs and T Cells The B7-H1 molecule is member of the B7 family of cosignaling molecules. It is upregulated on MDCs in ovarian cancer [8]. Signaling through B7-H1 facilitates tumor growth [80] by inducing apoptosis of effector T cells [81]. Furthermore, B7-H1 signals in the tumor environment induce IL-10 production by T cells, leading to immune suppression [8]. Blockade of B7-H1 enhances MDC-mediated T cell

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activation accompanied by downregulation of T cell IL-10 production and upregulation of T cell IL-2 and interferon γ (INFγ) [8]. It has been shown that B7-H1 signals on endothelial cells induce CD4+ CD25+ FOXP3+ Tregs [82]. Thus, blockade of B7-H1 signals on MDCs (and perhaps other cells) in the tumor environment may inhibit development or function of tumor-associated Tregs. This mechanism most likely contributes to the improved tumor immunity after B7-H1 blockade in vivo [8]. 12.5

VASCULAR LEUKOCYTES

A novel subset of antigen-presenting cells named vascular leukocytes that accumulate in human and murine ovarian cancer and other tumors have been described [83,84]. These myeloid lineage cells exhibit features of both dendritic cells as well as endothelial precursor cells. Vascular leukocytes are endowed with vasculogenic potential, because they can autonomously build blood vessels in vivo, and promote tumor growth [83,84]. Human vascular leukocytes are actually dendritic cells, as >97% are CD11c+ and express class II antigens among other molecules. They also express endothelium-specific markers. Vascular leukocytes are distinct from bona fide endothelial cells, which express no leukocyte markers and are also distinct from bona fide MDCs or PDCs. The immune function of vascular leukocytes in tumors remains to be fully characterized, but they are tolerogenic, induce IL-10-secreting T cells, and promote tumor growth. Thus, vascular leukocytes resemble tolerogenic dendritic cells that drive the development of Tregs. New evidence now indicates that vascular leukocytes also contribute to Treg expansion in cancer (Coukos G and others, unpublished data). Thus, means to prevent T cell–vascular leukocyte interactions could reduce Treg numbers or function, which could be exploited therapeutically. 12.6

T-REGULATORY CELLS AND TUMOR VACCINES

Tumor vaccines have enjoyed some limited successes, but their utility remains modest. Although some vaccines induce TAA-specific immunity, significant, longlasting clinical benefits from these vaccines have seldom been observed. The prior discussions of tumor-associated Tregs now help provide mechanisms for the failure of at least some vaccines. Effective cancer vaccines must overcome the immunosuppressive tumor microenvironment to help stimulate a potent antitumor immune response. However, more recent data suggest that cancer vaccines may contribute to immunosuppression by expanding tumor microenvironmental Tregs [7]. These data are consistent with other recent data showing that certain subsets of dendritic cells can also expand various Treg subsets [8,9]. Taken together, these data imply that the capacity of cancer vaccines to induce effective antitumor immunity may be impaired by the simultaneous expansion of vaccine-induced Tregs.


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As we begin to combine Treg depletion with active cancer vaccination, additional issues must be dealt with for optimal effectiveness. TAA-specific CD4+ CD25+ Tregs have been identified [85]. Treg depletion should attempt to deplete TAA-specific Tregs and not other Tregs so as to perturb homeostatic immunity the least. Treg depletion could also induce pathologic autoimmunity. In this regard, Treg depletion may be necessary, but not sufficient to induce autoimmunity [86]. The timing of Treg depletion in relation to active vaccination in a mouse model for colon cancer greatly influenced vaccine-engendered immunity. Treg depletion (using PC61 antibody) was most effective when given at the time of active vaccination [87]. Timing of Treg depletion likely will vary depending on factors related to the balance of TAA-specific effector cells versus Tregs in an individual, the agent used to deplete Tregs, the specific tumor type, and other factors. The potential for depletion of bystander cells was demonstrated using weekly Ontak dosing (Curiel and others, unpublished results). Future strategies to deplete Tregs should focus on mechanisms to target Tregs selectively, which will be facilitated by identification of Treg-specific molecules. Finally, our work and that of most investigators in both mice and humans have all focused on effects of CD4+ CD25+ Tregs. Nonetheless, additional regulatory cell populations exist, including CD8+ Tregs [68], immature myeloid cells [88], and B7-H4+ myeloid cells [89]. Additional regulatory populations such as NKT cells [90,91] have been proposed to be immunopathogenic in cancer, and likely will also be shown to play roles in immune suppression and the lack of efficacy of tumor vaccines.

12.7


Malignant tumors express associated antigens that should make them objects of immune attack. Nonetheless, spontaneous immune clearance of established cancer is rare. Evidence demonstrates that cancers utilize active mechanisms to block host antitumor immunity. Significant evidence implicates CD4+ CD25+ T-regulatory cells (Tregs) as important mediators of active immune evasion in cancer. Animal models for cancer demonstrate that Treg depletion improves endogenous antitumor immunity and the efficacy of active immunotherapy. Thus, inhibiting Treg function could improve human cancer immunotherapy, where success has been modest to date. We propose four means to block Treg activity: depletion of Tregs, interference with Treg trafficking, inhibiting Treg differentiation, or blocking Treg function. The fusion toxin denileukin diftitox (Ontak) can reduce Treg numbers in blood of some patients with cancer. We have discussed strategies to block Treg activity and presented some of our preliminary data in this regard. Combining Treg depletion with active vaccination and other approaches poses additional challenges that must be addressed. Disrupting tumor-mediated mechanisms hindering host immunity is a novel approach to tumor immunotherapy. More recent evidence suggests that CD4+ CD25+ T regulatory cells (Tregs) significantly impair tumor-specific immunity and contribute to poor survival in

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some cancers. Consequently, inhibiting Treg function could improve cancer therapy, including immunotherapy, which is corroborated by studies in animal models for cancer and Treg depletion. We proposed four general strategies to inhibit Treg function: (1) reducing their numbers, or interfering with their (2) function, (3) trafficking, or (4) differentiation. Reports from small human clinical trials demonstrate that killing Tregs is associated with augmented endogenous or vaccine-induced immunity, including tumor-specific immunity in some cases. Thus, Treg depletion may be a useful adjunct to some therapies. Which of these means to inhibit immunopathologic Treg function will prove most effective remains to be established. For example, Treg depletion might encourage rapid local Treg regeneration. Blocking Treg trafficking might inadvertantly reduce ingress of beneficial effector cells. The fusion protein denileukin diftitox (Ontak) has received attention as a potential agent to deplete functional Treg. As Ontak theoretically depletes any T cell bearing IL-2 receptors (including effector T cells), its utility may be limited in some settings. Additional agents to deplete Tregs are now under study, including cyclophosphamide or agents specifically targeting Tregs. Additional issues to address when to consider combining Treg depletion with active vaccination include the timing of Treg depletion versus vaccination, consideration for depletion of TAA-specific Tregs, collateral damage to newly induced TAA-specific effector cells, and potential for development of pathologic autoimmunity. The role of additional suppressor cell populations in tumor immunopathology aside from CD4+ CD25+ Tregs also merits further study. REFERENCES 1. Pardoll D: Does the immune system see tumors as foreign or self? Annu Rev Immunol 21:807–839 (2003). 2. Zou W: Immunosuppressive networks in the tumour environment and their therapeutic relevance, Nat Rev Cancer 5:263–274 (2005). 3. Shimizu J, Yamazaki S, Sakaguchi S: Induction of tumor immunity by removing CD25+ CD4+ T cells: A common basis between tumor immunity and autoimmunity, J Immunol 163:5211–5218 (1999). 4. Tanaka H, Tanaka J, Kjaergaard J, Shu S: Depletion of CD4+ CD25+ regulatory cells augments the generation of specific immune T cells in tumor-draining lymph nodes, J Immunother 25:207–217 (2002). 5. Steitz J, Bruck J, Lenz J, Knop J, Tuting T: Depletion of CD25(+ ) CD4(+ ) T cells and treatment with tyrosinase-related protein 2-transduced dendritic cells enhance the interferon alpha-induced, CD8(+ ) T-cell-dependent immune defense of B16 melanoma, Cancer Res 61:8643–8646 (2001). 6. Stutmuller RP, van Duivenvoorde LM, van Elsas A, Schumacher TN, Wildenberg ME, Allison JP, Toes RE, Offringa R, Melief CJ: Synergism of cytotoxic T lymphocyte-associated antigen 4 blockade and depletion of CD25(+ ) regulatory T cells in antitumor therapy reveals alternative pathways for suppression of autoreactive cytotoxic T lymphocyte responses, J Exp Med 194:823–832 (2001).

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13 TUMOR VACCINATION AFTER AUTOLOGOUS HSCT: WHAT HAS BEEN LEARNED FROM EXPERIMENTAL MODELS Weiqing Jing and Bryon D. Johnson

13.1

INTRODUCTION

The use of vaccines for the treatment of cancer has been a clinically challenging endeavor for several reasons, including the fact that unlike infectious agents that contain foreign antigens capable of inducing relatively strong immune responses, cancer cells are of self-origin and therefore may be capable of inducing only weak immunity. Since cancer is a disease of self-origin, and immune regulatory networks are in place to prevent self-reactivity [1], cancer vaccines need to induce a response that is strong enough to overcome the immune regulation. Another problem is that unlike the use of vaccines for infectious agents, which are given to prevent infection, cancer vaccines are administered in the face of active disease. Cancer cells themselves may produce factors that suppress infiltrating immune effectors [2], or the growing cancer may activate immune regulatory cells capable of suppressing the immune system [3]. Therefore, unless used in the context of minimal disease and altered immune regulation, vaccines for cancer may be destined to fail. High-dose therapy followed by hematopoietic stem cell transplantation (HSCT) is an attractive Cancer Vaccines and Tumor Immunity Edited by Rimas Orentas, James W. Hodge, and Bryon D. Johnson Copyright © 2008 John Wiley & Sons, Inc. 205

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platform for cancer vaccine administration since it can achieve both of these goals (reduction of disease to minimal levels and altered immune regulation). Clinical and experimental data have also suggested that the lymphopenic state occurring early after HSCT might allow for better induction and skewing of immunity toward tumor reactivity [4–7]. Vaccinating early after HSCT has become of particular interest since the mid-1990s as researchers and clinicians strive to improve the efficacy of cancer vaccines. This chapter briefly explores what has been learned from mouse syngeneic HSCT studies regarding early posttransplant vaccination for cancer. 13.2 ANIMAL MODELS OF AUTOLOGOUS HSCT: WHAT WE HAVE LEARNED The primary goal of autologous HSCT for cancer has been to eliminate disease using high doses of chemotherapy and/or radiotherapy. Unfortunately, although tumor cell numbers can be reduced to undetectable levels, patients often relapse from residual microscopic disease since there is a lack of graft-versus-tumor immune reactivity in the autologous HSCT setting. Results from experimental mouse studies have shown that cancer vaccines can elicit tumor immunity when administered after syngeneic HSCT [4,8–17], but a number of factors can influence the effectiveness of vaccination, including the level of immune reconstitution, the presence of immune regulatory cells, and tumor burden (i.e., numbers of remaining tumor cells). Each of these factors has been or is currently being examined in mice administered syngeneic HSCT. Information learned from these experimental studies is briefly discussed in the following sections. 13.3

IMMUNE RECONSTITUTION

T lymphocytes are important effectors for the generation of vaccine-induced antitumor responses. While data from investigators have suggested that it may be easier to skew T cell immune reactivity toward cancer after HSCT [4,5], the ability to generate an immune response immediately after HSCT may be difficult because of the severe lymphopenia that typically occurs after high-dose conditioning. When mice are conditioned with lethal total-body irradiation (TBI), there is a period of profound lymphopenia that lasts for approximately 3 weeks after HSCT (Fig. 13.1a). T cell numbers are severely reduced during this time period, and their numbers remain below pretransplant levels even 50 days after transplant (Fig. 13.2). T cell reconstitution early after transplant occurs primarily through thymus-independent mechanisms, and since mouse bone marrow (BM) contains relatively few T cells ( ∼1–4%), the first 3 weeks after HSCT are dominated by small numbers of residual host (or recipient) CD4+ T cells. Since thymic reconstitution does not occur until around 2 weeks after transplant (Fig. 13.1b), relatively few BM-derived T cells


(a)

207

(b)

Figure 13.1. Lymphoid tissue reconstitution after syngeneic HSCT: (a) splenic reconstitution; (b) thymic reconstitution. AKR/J recipients were lethally irradiated and transplanted 24 h later by intravenous injection with 107 syngeneic AKR/Cum bone marrow cells. Cell counts were done on spleens (a) and thymuses (b) collected before HSCT (Pre) or on days 3, 7, 14, 21, 28, and 50 after HSCT. The data represent the averages and standard deviations of three mice analyzed at each timepoint.

Figure 13.2. T cell reconstitution after syngeneic HSCT (bone marrow only). Lethally irradiated AKR/J recipients (Thy1.1+ ) were transplanted with 107 syngeneic AKR/Cum (Thy1.2+ ) bone marrow cells. Phenotypic analysis of spleens was done before HSCT (Pre) or at the indicated timepoints after HSCT. Recipient T cells could be distinguished from bone marrow–derived (donor) T cells because of the Thy1 allelic difference. The data represent the averages of three mice analyzed at each timepoint.

can be detected in the spleen until 3–4 weeks after HSCT. Other investigators [4,9] have similarly reported this pattern of cellular reconstitution in mice. In order to prevent posttransplant tumor relapse using cancer vaccines, one might hypothesize that vaccination should be initiated as soon as possible after HSCT. However, the data in Fig. 13.2 suggest that T cell numbers during the first 3 weeks following

208


HSCT may be suboptimal for the generation of effective vaccine-induced tumor immunity. To support this, Teshima and coworkers [16] found that immune reconstitution was critical for the induction of tumor immunity elicited by tumor vaccination. These observations suggested that if T cell reconstitution could be accelerated, then the efficacy of early posttransplant tumor vaccination could be improved. Our laboratory has examined the impact of supplementing mouse BM with a source of T cells (splenocytes), or infusing purified T cells early (3 days) after HSCT, in an effort to improve the efficacy of tumor vaccination early posttransplant. When we examined T cell reconstitution in lethally irradiated mice given BM supplemented with 20 million splenocytes (containing ∼ 6 million T cells), we found that although total splenic T cell numbers are not dramatically altered during the first 3 weeks after transplant (cf. Fig. 13.3 vs. Fig. 13.2), the majority of T cells during this time period are from the infused splenocytes (Fig. 13.3). Importantly, when a vaccine for neuroblastoma is administered to syngeneic A/J mice on days 7 and 14 after HSCT, only those mice that receive an infusion of syngeneic T cells are able to resist a challenge of live neuroblastoma cells 21 days after HSCT (Fig. 13.4). Thus, for this particular tumor and tumor vaccine, there is an absolute requirement for T cell adoptive transfer in order to induce tumor-protective immunity early after HSCT. Interestingly, as

Figure 13.3. T cell reconstitution after syngeneic HSCT (BM + splenocytes). C57BL/6 (Thy1.2+ CD45.2+ ) mice were lethally irradiated and 24 h later injected with 107 bone marrow cells from congenic B6.PL-Thy1a mice (Thy1.1+ CD45.2+ ) plus 2 × 107 splenocytes from congenic B6-CD45.1 mice (Thy1.2+ CD45.1+ ). Mice were killed at the indicated timepoints and the spleens collected for analysis. Nucleated cell numbers were counted, and phenotypic analysis of spleens was done before HSCT (Pre) or at the indicated timepoints after HSCT. Recipient, donor bone marrow (BM)–derived, and donor spleen (Spln)– derived T cells could be distinguished from one another by the Thy1 and CD45 allelic differences. The data represent the averages of six mice analyzed at each timepoint.


209

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60 50 40 p = 0.0069

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Figure 13.4. Protective tumor vaccine-induced immunity requires the presence of T cells in the graft. Lethally irradiated A/J mice were transplanted with 107 syngeneic BM cells, and 3 days later given or not given 5 × 106 purified T cells. One week after HSCT, recipients were vaccinated twice weekly with a cell-based neuroblastoma vaccine consisting of tumor cells that had been genetically modified to express CD54, CD80, CD86, and CD137 L. One week after the last vaccination, the mice were challenged with 104 live parental AGN2a neuroblastoma cells and followed for tumor development. The data (n = 4 mice/group) are representative of two separate experiments.

noted by Borrello and colleagues [4], recovery of lymphoid cell numbers toward pretransplant levels is not required for generation of protective vaccine-induced immunity. In an effort to examine whether early post-HSCT tumor vaccination might facilitate better tumor-specific T cell responses as compared to later vaccination, we administered two weekly neuroblastoma vaccines at different times after HSCT (started on days 7, 14, 21, or 28), and tested purified immune T cells for neuroblastoma reactivity in interferon-gamma (INFγ) ELISPOT assays. The highest frequencies of tumor-reactive CD4+ and CD8+ T cells in these assays were observed in mice vaccinated on days 7 and 14 (Fig. 13.5), and as compared to historical data from nontransplanted/vaccinated mice, frequencies of tumor-reactive T cells were higher at all post-HSCT vaccine timepoints. These data are in agreement with observations by other investigators who have shown that altered T cell homeostatic mechanisms early after HSCT allow for the skewing of T cell immune responses toward tumor antigens [4]. While it is clear that T cells are critical for the generation of effective vaccine-induced antitumor immunity, experimental data have indicated that other immune cells can contribute to, or play an important role in, antitumor immune responses depending on the nature of the target antigens and the type of vaccine used. Earlier experimental HSCT studies focused on generation of antiidiotypic immunoglobulin (Ig) responses to tumor-derived surface Ig as a strategy to treat

210


IFN-gamma Spots per 105 Cells

1600 Non-transplanted Day 28, 35 Day 21, 28 Day 14, 21 Day 7, 14

1400 1200 1000 800 600 400 200 0

CD4+ T Cells

CD8+ T Cells

Figure 13.5. Tumor vaccination early after HSCT induces the highest frequencies of tumor-reactive T cells. Lethally irradiated A/J mice were transplanted with syngeneic bone marrow cells plus 2 × 107 syngeneic splenocytes. At the indicated timepoints after HSCT (see legend in the diagram), the mice were vaccinated with a cell-based neuroblastoma vaccine consisting of tumor cells that had been genetically modified to express CD54, CD80, CD86, and CD137 L. Five days after the last vaccination, CD8+ and CD4+ splenic T cells were isolated and analyzed for frequencies of neuroblastoma-specific IFNγ-secreting cells by ELISPOT. CD4 cell reactivity was assessed using a MHC class II+ neuroblastoma cell line (AGN2a-CIITA) that was created by transfecting AGN2a neuroblastoma cells with a plasmid encoding CIITA. A group of nontransplanted/vaccinated mice was included for comparison. The data represent the averages and standard deviations of triplicate wells.

B cell lymphoma [12,13]. These studies showed that tumor-protective Ig responses could be generated as early as 3 weeks after HSCT by vaccination [13], demonstrating that Ig responses can play a role in post-HSCT immunity to cancer if appropriate antigenic targets are present on the tumor. Chen and colleagues [10] showed that early post-HSCT vaccination with tumor-derived chaperone-rich cell lysates, which are composed of heatshock proteins, could induce immunity to a mouse leukemia. Natural killer (NK) cells as well as T cells were involved in the antitumor immunity. These results point out that both adaptive and innate immunity can be induced by early post-HSCT vaccination, and since there is crosstalk between these two arms of the immune system, efforts to activate both might provide additive or synergistic effects. In summary, despite the lymphopenia and depressed T cell numbers early after myeloablative HSCT (Figs. 13.1 and 13.3), experimental animal data have shown that if sufficient numbers of mature T cells are present in the graft, cancer vaccines given early post-HSCT (within the first 2 weeks) can induce tumor-protective immunity. Furthermore, early post-HSCT vaccination may allow for the skewing of T cell immunity in favor of tumor reactivity.

PRESENCE OF IMMUNE REGULATORY CELLS

13.4

211

PRESENCE OF IMMUNE REGULATORY CELLS

With increasing numbers of studies indicating that regulatory T (Treg) cells play a role in the suppression of immunity to cancer [3], investigators are examining whether inhibiting these cells can increase the efficacy of cancer immunotherapies. While different populations of Treg cells have been described, the best-characterized population consists of CD4+ cells that coexpress the transcription factor, Foxp3, and the alpha chain of the IL-2 receptor, CD25 [18]. One consequence of the myeloablative treatment used in preparation for HSCT is the elimination of Treg cells in addition to elimination of potential immune effector cells. Our laboratory has examined CD4+ Foxp3+ cells in lethally irradiated mice given syngeneic HSCT with BM and 20 million added splenocytes. Only small numbers of CD4+ Foxp3+ cells survive the lethal conditioning (Fig. 13.6a), and numbers remain low until 3 weeks after HSCT. Notably, the majority of CD4+ Foxp3+ cells during the first 4 weeks after HSCT are derived from the infused splenocytes, and new thymus-derived CD4+ Foxp3+ cells do not appear until days 21–28 (not shown). Despite the depressed numbers of CD4+ Foxp3+ cells during the first 2 weeks after HSCT, increased percentages of CD4+ cells express Foxp3 (Fig. 13.6b), in large part due to increased percentages of Foxp3+ cells originating from the infused splenic CD4+ cells (not shown). On the basis of these data, we speculate that CD4+ Foxp3+ cells might have some impact on vaccine-induced immunity early post-HSCT. More recent preliminary results suggest that depletion of CD25+ cells from T cells (splenocytes) added to BM may increase vaccine efficacy against a mouse neuroblastoma (Fig. 13.7), but these experiments will need to be repeated to see if the results are statistically significant. There is some concern that manipulation of Treg cells will increase the risk for development of autoimmune disease, particularly in the HSCT setting where Treg cells are already being impacted by the conditioning regimens. However, despite the relatively low numbers of CD4+ Foxp3+ cells early after HSCT, development of autoimmune disease has not been an issue following syngeneic HSCT in mice. Perhaps a small number of radioresistant Treg cells are adequate to prevent development of autoimmune immune disease, as has been suggested by Benard and colleagues [19]. In addition, since the time interval between transplant and thymic reconstitution in mice is relatively short, small numbers of Treg cells early post-HSCT may be sufficient to blunt autoreactivity until new thymus-derived Treg cells emerge. Future studies using Treg-deficient, thymus-deficient mice as recipients of Treg cell-depleted HSCT should be able to address these possibilities. The role of Treg cells in cancer immunity early after HSCT is just beginning to be explored. Whether Treg cells impact vaccine-induced immunity early after HSCT, and what the magnitude of their impact might be, can be examined using appropriate experimental animal models. While myeloablative therapy might allow for improved vaccine efficacy in part due to reduction in Treg cell numbers, it will be important to know whether further inhibition of Treg cells increases vaccination efficacy and/or increases the risk for development of autoimmune disease.


CD4+Foxp3+ Cells per Spleen (105)

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% of CD4+ cells expressing Foxp3

Pre

35 30 25 20

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7 14 21 Day after HSCT (a)

* P < 0.05 ** P < 0.01 *** P < 0.001 versus Pre * ***

28

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28

35

(b)

Figure 13.6. CD4+ Foxp3+ T cells after HSCT. C57BL/6 mice were lethally irradiated and 24 h later injected with syngeneic bone marrow cells plus 2 × 107 splenocytes. Mice were killed at the indicated timepoints and the spleens collected for analysis. Nucleated cell numbers were counted, and the remaining splenocytes were analyzed by flow cytometry to assess absolute numbers of CD4+ cells coexpressing Foxp3 (a) and the percentages of CD4+ cells coexpressing Foxp3 (b). The data represent the mean values of six mice combined from two experiments (n = 3 mice analyzed in each experiment). Standard deviations and p values are shown in panel (b).

13.5

TUMOR BURDEN

Vaccination for infectious disease has taught us that vaccines, even for potent non-self-antigens, are likely to fail in the face of established disease. The lack of clinical success using cancer vaccines to treat patients with “bulky” disease confirms this. Using experimental models, investigators have shown that vaccination against “established disease” can be effective if given early after disease inoculation, while vaccination at later timepoints only delays growth [20]. The

TUMOR BURDEN

213

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CD25-depleted splenocytes

20

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Figure 13.7. Depletion of CD25+ cells from T cells added to BM appears to increase vaccine efficacy. Lethally irradiated A/J mice were transplanted with syngeneic bone marrow cells plus 2 × 107 nonseparated splenocytes or splenocytes depleted of CD25+ by immunomagnetic depletion. The transplanted mice were vaccinated on days 7 and 14 after HSCT with irradiated neuroblastoma cells that had been genetically modified to express CD54, CD80, CD86, and CD137 L. All mice were then challenged 3 weeks after the last vaccination with 5 × 106 live neuroblastoma (AGN2a) cells. The groups consisted of six mice per group.

ability of myeloablative therapy to reduce tumor burdens to nondetectable levels, or nearly nondetectable levels, is why the early post-HSCT time period is attractive for vaccinating against cancer. While animal studies have provided data showing that vaccines can induce protective tumor immunity early after syngeneic HSCT, studies have shown that even in the post-HSCT setting it can be difficult to eliminate established disease by vaccination [8,10]. Part of the difficulty in interpreting the results of these studies is that mouse tumor cell lines are sometimes highly aggressive when inoculated in vivo, providing a narrow “window of opportunity” in which to vaccinate tumor-bearing animals. However, a clear message from the experimental observations is that vaccination should be initiated as early as possible after HSCT. Chen and colleagues [10] found that the most effective early post-HSCT vaccination of mice with established leukemia required transplant of “immune” BM and splenocytes from prevaccinated syngeneic donors. Mice that had been administered nonimmune HSCT showed no benefit from tumor vaccination during the first week after transplant [10], confirming similar observations by other investigators [4]. These results suggest that a multifaceted approach combining myeloablative therapy, HSCT, adoptive immunotherapy (with vaccine-primed T cells collected pretransplant), and vaccination may be required to effectively eliminate residual disease for some tumors. It is noteworthy that this type of multifaceted approach is already being explored clinically, with interesting results [21].

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CONCLUSIONS AND FUTURE CONSIDERATIONS

Experimental mouse studies have provided information regarding the ability to vaccinate early after HSCT setting, have shown that the presence of T cells is crucial to the success of early post-HSCT tumor vaccination, and have demonstrated that tumor burden can impact on vaccine efficacy. Lessons learned from the experimental work are being applied in the clinic, but there is a need for additional preclinical work. Some important questions that still need to be addressed include 1. What is the frequency of vaccination needed to induce optimal antitumor immunity early after HSCT? Are repeated vaccinations needed to maintain optimal tumor immunity? 2. Can post-HSCT vaccination be optimized by using vaccine preparations that activate both innate and adaptive immunity? 3. Can further perturbation of immune regulatory mechanisms improve vaccine efficacy without toxicity? 4. How can adoptive immunotherapy be most effectively combined with posttransplant vaccination, and what mechanisms are involved? 5. Is the generation of T cell memory important to vaccine-induced immunity post-HSCT, and if so, how can memory be most effectively generated? REFERENCES 1. Paust S, Cantor H: Regulatory T cells and autoimmune disease, Immunol Rev 204:195 (2005). 2. Kim R, Emi M, Tanabe K: Cancer cell immune escape and tumor progression by exploitation of anti-inflammatory and pro-inflammatory responses, Cancer Biol Ther 4:924 (2005). 3. Orentas RJ, Kohler ME, Johnson BD: Suppression of anti-cancer immunity by regulatory T cells: Back to the future, Semin Cancer Biol 16:137 (2006). 4. Borrello I, Sotomayor EM, Rattis F-M, Cooke SK, Gu L, Levitsky HI: Sustaining the graft-vesus-tumor effect through posttransplant immunization with granulocyte-macrophage colony-stimulating factor (GM-CSF)-producing vaccines, Blood 95:3011 (2000). 5. Mackall CL, Bare CV, Granger LA, Sharrow SO, Titus JA, Gress RE: Thymicindependent T cell regeneration occurs via antigen-driven expansion of peripheral T cells resulting in a repertoire that is limited in diversity and prone to skewing, J Immunol 156:4609 (1996). 6. Dummer W, Niethammer AG, Baccala R, Lawson BR, Wagner N, Reisfeld RA, Theofilopoulos AN: T cell homeostatic proliferation elicits effective antitumor autoimmunity, J Clin Invest 110:185 (2002). 7. Mori S, Kocak U, Shaw JL, Mullen CA: Augmentation of post transplant immunity: Antigen encounter at the time of hematopoietic stem cell transplantation enhances antigen-specific donor T-cell responses in the post transplant repertoire, Bone Marrow Transplant 35:793 (2005).

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8. Asavaroengchai W, Kotera Y, Koike N, Pilon-Thomas S, Mule JJ: Augmentation of antitumor immune responses after adoptive transfer of bone marrow derived from donors immunized with tumor lysate-pulsed dendritic cells, Biol Blood Marrow Transplant 10:524 (2004). 9. Asavaroengchai W, Kotera Y, Mule JJ: Tumor lysate-pulsed dendritic cells can elicit an effective antitumor immune response during early lymphoid recovery, Proc Natl Acad Sci USA 99:931 (2002). 10. Chen X, Zeng Y, Li G, Larmonier N, Graner MW, Katsanis E: Peritransplantation vaccination with chaperone-rich cell lysate induces antileukemia immunity, Biol Blood Marrow Transplant 12:275 (2006). 11. Dunussi-Joannopoulos K, Krenger W, Weinstein HJ, Ferrara JL, Croop JM: CD8+ T cells activated during the course of murine acute myelogenous leukemia elicit therapeutic responses to late B7 vaccines after cytoreductive treatment, Blood 89:2915 (1997). 12. Kwak LW, Campbell M, Levy R: Idiotype vaccination post-bone marrow transplantation for B-cell lymphoma: Initial studies in a murine model, Cancer Detect Prev 15:323 (1991). 13. Kwak LW, Campbell MJ, Zelenetz AD, Levy R: Combined syngeneic bone marrow transplantation and immunotherapy of a murine B-cell lymphoma: active immunization with tumor-derived idiotypic immunoglobulin, Blood 76:2411 (1990). 14. Pawlowska AB, Hashino S, McKenna H, Weigel BJ, Taylor PA, Blazar BR: In vitro tumor-pulsed or in vivo Flt3 ligand-generated dendritic cells provide protection against acute myelogenous leukemia in nontransplanted or syngeneic bone marrow-transplanted mice, Blood 97:1474 (2001). 15. Sato K, Torimoto Y, Tamura Y, Shindo M, Shinzaki H, Hirai K, Kohgo Y: Immunotherapy using heat-shock protein preparations of leukemia cells after syngeneic bone marrow transplantation in mice, Blood 98:1852 (2001). 16. Teshima T, Mach N, Hill GR, Pan L, Gillessen S, Dranoff G, Ferrara JL: Tumor cell vaccine elicits potent antitumor immunity after allogeneic T-cell-depleted bone marrow transplantation, Cancer Res 61:162 (2001). 17. Zeis M, Frenzke H, Schmitz N, Uharek L, Steinmann J: Idiotype protein-pulsed dendritic cells produce strong anti-myeloma effects after syngeneic stem cell transplantation in mice, Bone Marrow Transplant 29:213 (2002). 18. Sakaguchi S: Naturally arising Foxp3-expressing CD25+ CD4+ regulatory T cells in immunological tolerance to self and non-self, Nat Immunol 6:345 (2005). 19. Benard A, Ceredig R, Rolink AG: Regulatory T cells control autoimmunity following syngeneic bone marrow transplantation, Eur J Immunol 36:2324 (2006). 20. Dunussi-Joannopoulos K, Weinstein HJ, Nickerson PW, Strom TB, Burakoff SJ, Croop JM, Arceci RJ: Irradiated B7-1 transduced primary acute myelogenous leukemia (AML) cells can be used as therapeutic vaccines in murine AML, Blood 87:2938 (1996). 21. Rapoport AP, Stadtmauer EA, Aqui N, Badros A, Cotte J, Chrisley L, Veloso E, Zheng Z, Westphal S, Mair R, Chi N, Ratterree B, Pochran MF, Natt S, Hinkle J, Sickles C, Sohal A, Ruehle K, Lynch C, Zhang L, Porter DL, Luger S, Guo C, Fang HB, Blackwelder W, Hankey K, Mann D, Edelman R, Frasch C, Levine BL, Cross A, June CH: Restoration of immunity in lymphopenic individuals with cancer by vaccination and adoptive T-cell transfer, Nature Med 11:1230 (2005).

14 VACCINES IN THE SETTING OF HEMATOPOIETIC STEM CELL TRANSPLANTATION Ronald Gress and Claude Sportes

14.1

INTRODUCTION

Effective treatment of metastatic solid tumors remains a clinical challenge. Escalating doses of available chemotherapeutic agents have not, in general, produced high cure rates in metastatic breast cancer [1], for example, and much of the current emphasis is placed on the introduction of more targeted agents. It is in this setting that interest exists in developing alternative, immune-based strategies such as tumor vaccines, adoptive transfer of in vitro generated effector T cell populations, and allogeneic hematopoietic stem cell transplantation (HSCT). While the limitations of standard chemotherapeutic agents are recognized, those that provide efficacy in disease control also typically deplete cells of the immune system. On initial consideration, this side effect may seem undesirable for the development of new treatment strategies that are immune-based, but characterization of the regenerating immune response has provided insights indicating that immune-depleting chemotherapy might act as a platform for immunotherapies such as those employing tumor-specific vaccines or adoptive T cell transfer. That perspective is based Cancer Vaccines and Tumor Immunity Edited by Rimas Orentas, James W. Hodge, and Bryon D. Johnson Copyright © 2008 John Wiley & Sons, Inc. 217

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on the understanding and derivative hypotheses that dose-intensive chemotherapy can lengthen the progression-free survival period [2–4], thus allowing a “time window” for a slow-acting therapy such as vaccination to be effective, that maximally decreasing the patient’s tumor burden may increase the effectiveness of immunotherapy [5,6] by mechanisms including decreases in tumor-induced immune suppressive effects [7–9], and that providing tumor antigen exposure following immune depletion in the form of repeated immunizations may take advantage of the pattern of immune reconstitution at early timepoints (antigen-driven peripheral expansion of T cells) and allow for development of a T cell repertoire biased toward tumor antigens. In addition, unaltered T cells in the form of reinfused lymphocytes collected prechemotherapy can be used to augment T cell responses to antigen following immunoablative chemotherapy, perhaps especially in the setting of primed T cell transfer.

14.2

TRANSPLANTATION, TUMOR MASS, AND TIME

Several animal models of immune-mediated tumor eradication have shown that preventing the establishment of a tumor is much more readily achievable than eradicating an established tumor. This suggests that a lower tumor burden increases tumor susceptibility to immune effector cells. The advantage of a lower tumor burden in immunotherapy has also been demonstrated clinically in the postallogeneic HSCT setting using donor lymphocyte infusions (DLIs) in patients with hematologic malignancies. Chronic myelogenous leukemia (CML) patients with cytogenetic or hematologic relapse have a significantly higher complete response rate and improved outcome with DLI compared to those with greater tumor burden [5,6]. A second consideration is immune suppression induced by the tumor itself. Such suppression may in part be directly related to tumor bulk and thus may be improved with a greater reduction in tumor mass through high-dose chemotherapy. Experimental evidence points to a specific immunosuppressive effect of large tumor burdens, including decreases in specific antitumor T lymphocyte activity as the tumor burden increases [7]. Depressed specific antitumor responses have been described in tumors expressing the MUC1 antigen, via either direct suppression [8] or induction of lymphocyte apoptosis [9]. It is likely that some of these findings may also be relevant to other tumor antigens. Immune strategies may require a prolonged period for effectiveness. This was illustrated in CML patients following allogeneic HSCT who showed a progressive conversion from cytogenetic to molecular remission over a period of months [10]. A more recent study in patients with renal cell carcinoma also demonstrates the need for prolonged exposure to immune effectors for in vivo tumor eradication [11]. A dose-intense regimen that can result in a prolongation of the time to tumor progression may increase the time window in which an immune-based strategy can have its effect. It may be advantageous to decrease the tumor burden with dose-intense therapy, not only to decrease the tumor mass but also to potentially increase the efficacy of the subsequent immune intervention by decreasing

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antitumor immune suppression and by widening the time window for a slow-acting immune therapy. Therefore, dose-intense chemotherapy followed by immune depletion may serve most importantly as a platform for subsequent immune-based therapies. 14.3

TRANSPLANTATION AND A WINDOW OF SURVIVAL: AN EXAMPLE

In 1996 we opened a clinical protocol seeking to improve the survival of patients with metastatic breast cancer. Treatment consisted of three to seven cycles of continuous 72-h infusion of paclitaxel (160 mg/m2 ) and daily cyclophosphamide (900 mg/m2 × 3) (to maximum clinical response). The second cycle was used for stem cell mobilization purposes. For patients not previously treated with an anthracycline, four cycles of doxorubicin (60 mg/m2 )/cyclophosphamide (600 mg/m2 ) were administered, and local control was approached using surgery and /or radiation therapy as clinically indicated. The high-dose chemotherapy consisted of melphalan (160 mg/m2 )/etoposide (1800 mg/m2 ) followed by autologous peripheral blood stem cell transplantation. In order to proceed with the high-dose chemotherapy, patients had to at least achieve a partial remission (defined as 50% or more reduction of measurable disease). Hormonal therapy and/or bisphosphonate therapy were given as clinically indicated. At the time of last analysis [12], progression-free and overall survival probabilities at 24 months posttransplant for stages IIIA, IIIB, and IV transplanted patients were 82/81/42%, 100/94%, and 68%, respectively. The median progression-free and overall survival times from entry on study for stage IV patients were 15.3 and 38.1 months, respectively. For stage III patients, the median progression-free survival has not been reached. In both groups, the window of survival was sufficient to allow introduction of immune-based therapies in an attempt to develop a curative therapy. 14.4 ALTERNATIVES TO HIGH-DOSE CHEMOTHERAPY FOR CONTROLLING DISEASE: T CELL DEPLETION OUTSIDE THE SETTING OF HIGH-DOSE CHEMOTHERAPY In other allogeneic HSCT protocols, the combination of fludarabine and cyclophosphamide (Flu-Cy) has been used successfully by our group to induce a state of profound immune depletion prior to nonmyeloablative T-cell-depleted HSCT in patients with recurrent metastatic breast cancer [13,14]. One or two cycles of fludarabine (30 mg/m2 ) and cyclophosphamide (600 mg/m2 ) for 4 consecutive days were used with the specific goal of depleting lymphocytes prior to administering the preparative regimen for transplantation. Patients received one or two courses of the Flu-Cy regimen on the basis of peripheral blood CD4 counts such that those patients with a CD4 count >50/mm3 after neutrophil recovery, 18–24 days after the first course, went on to receive a second course of Flu-Cy before progressing to the preparative regimen for transplant.

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The results comparing the cumulative immune depleting effect of the standard chemotherapy doses in the autologous transplant protocol described in the previous section (first 25 patients), and the immune-depleting effects of one course of Flu-Cy (first 12 patients) showed that the most striking differences involved the CD4+ T cell population. The absolute postdepletion counts were significantly lower after Flu-Cy than after several cycles of chemotherapy (44/mm3 vs. 161/mm3 , p < .0001), although the pretreatment CD4 counts were initially higher in the former group (612/mm3 vs. 436/mm3 , p = .023). Notably, the relative CD4 depletion (“Post”–“Pre”/“Pre”) obtained after one cycle of Flu-Cy was significantly greater with Flu-Cy (88% vs. 78%, p = .003). Thus, the biology of T cell regeneration, which contributes to the strategy of tumor vaccine administration in HSCT, also applies to T-cell-depleting nonmyeloablative chemotherapy.

14.5 ANTIGEN EXPOSURE POSTIMMUNE DEPLETION: T CELL REPERTOIRE RECONSTITUTION Experimental and clinical studies [15–21] of posttransplantation immune reconstitution have defined two pathways of T-cell regeneration following T-cell-depleting chemotherapy and autologous HSCT (Fig. 14.1). In murine models, in which the

CD4+ T Cell Regeneration in Humans Human

CD 45 RO

Children

CD 45 RA

CD 45 RO

Adults

CD 45 RA

Figure 14.1.

CD4+

T cell regeneration in humans.

ANTIGEN EXPOSURE POSTIMMUNE DEPLETION: T CELL REPERTOIRE RECONSTITUTION

221

existence of two primary (and only two primary) pathways of T cell regeneration following acute loss of peripheral T cell populations was demonstrated, it was shown that a thymus-dependent pathway was dominant in young animals, but that in the absence of a thymus, appreciable populations of competent T cell were generated by peripheral expansion of mature T cells; thus, mature T cells served as progenitors for reconstituting peripheral T cell populations by clonal expansion. Further, it was shown that this thymus-independent peripheral expansion pathway was influenced by antigen, and therefore was prone to skewing. The thymic pathway, on the other hand, was a source of T cell repertoire diversity. Because there is no known pathway for generation of appreciable T cell receptor diversity outside the thymus, identification of diversification is evidence of thymic activity. Perhaps most important in these early murine studies was the identification of CD45 isoform expression as a means to study T cell regeneration in humans in that the pathway gives rise to reconstituted T cell populations with characteristic phenotypes; the thymus-dependent pathway gives rise to naive T cell populations that primarily have a CD45RA isoform phenotype, while the peripheral expansion, thymus-independent pathway obligatorily gives rise to memory T cell populations with a CD45RO isoform phenotype. When CD45 isoform expression was used to study T cell regeneration in humans, it was the thymus-dependent pathway was found to predominate for regenerating lost CD45RA+ CD4+ T cells in children and young adults (Fig. 14.1), and that it allowed the reconstitution of a widely diverse repertoire of naive T cells. In initial studies in adults, it was found that peripheral expansion was the predominant pathway of T cell recovery, and no evidence for effective thymopoiesis was observed. In a later, long-term study involving multiple parameters of thymic activity (thymic size by imaging, identification of recent thymic emigrants by TREC analysis, assessment of T cell receptor diversity, and CD45 isoform expression by reconstituted T cell populations), it was found that thymopoiesis can contribute to reconstitution of lost T cell populations in a subset of adults with age as a controlling variable for its occurrence and timecourse—the younger the adult, the more likely that thymic recovery occurred, and that it occurred in these younger adults more quickly after cessation of T-cell-depleting chemotherapy [12,19]. The variation in thymic recovery by age reached its limit at approximately age 50, in that very few individuals beyond this point were observed to have numerically significant recovery of lost T cell populations even over 5 years of observation. This longer-term study confirmed the T cell repertoire skewing associated with T cell recovery via the peripheral expansion pathway. It also revealed a fundamental difference in the patterns of regeneration by CD4+ versus CD8+ T cells in that the latter underwent more profound peripheral expansion in some individuals and contributed long term to the numerical recovery of CD8+ T cell populations, whereas the long-term numerical recovery of CD4+ T cells populations was dependent on thymic activity (Figs. 14.2a and 14.2b). The difference in recovery patterns between CD4+ and CD8+ T cells finds its basis in the distinctly different homeostatic regulation of the two T cell subsets. Peripheral T cell populations exist in homeostatic cytokine-defined niches: IL-7

222


CD4+ T Cell Reconstitution (a) Thymic Size by Imaging

CD45 RA CD45 RO

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Post

6 mos

12 mos

24 mos

CD4 Cells Naïve Memory

36 mos

CD8+ T Cell Reconstitution (b) Thymic Size by Imaging

CD8 Cells Naïve Memory+Activated Pre

Post

6 mos

12 mos

24 mos

Figure 14.2. T cell reconstitution timecourse: (a)

CD4+

36 mos T cell reconstitution; (b) CD8+ T

cell reconstitution.

is the cytokine predominantly defining the niche for naive CD4+ and CD8+ T cell populations, while IL-15 predominantly defines the niche for central memory and activated effector CD8+ T cells. While IL-7 levels are not dynamically regulated to an appreciable extent (although they vary with CD4+ T cell number as a homeostatic regulator), IL-15 is a component of the inflammatory cascade, and therefore does vary, and is able to support large expansions of activated CD8+ T cells. At least in the mouse, homeostatic control of the central memory CD8+ T cell pool appears to be stabilized by TGFβ in that animals containing T cells incapable of responding to this cytokine have ever-expanding populations of CD8+ T cells with a central memory phenotype [23]. These studies of T cell reconstitution have thus identified three strategies for optimizing or exaggerating T cell responses to vaccine antigens in the setting of cancer therapy: (1) early vaccination postcessation of chemotherapy to take advantage of antigen-driven T cell reconstitution (with the antigen consisting of tumor specific vaccines), (2) adoptive infusion of T cells presensitized to specific antigens during the “early” vaccination time period to augment the progenitor pool available for peripheral expansion, and (3) “Late” vaccination to take advantage of the newly repaired T cell repertoire that emerges with thymic recovery in younger adults. While the rationale for administration of tumor vaccines during T cell regeneration after HSCT applies to both the autologous and allogeneic settings, certain unique features apply to the latter. One feature is that the immunotherapeutic effects inherent to allogeneic HSCT (i.e., graft–tumor reactivity) may be enhanced

VACCINE STRATEGIES: TARGET ANTIGENS AND TOLERANCE

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by administration of tumor antigens in the posttransplant period. Another unique feature regarding allogeneic HSCT is availability of the transplant donor to supply T cell populations that have been unaffected by tumor exposure, disease burden, or chemotherapy that can be administered as DLI, thereby providing a source of lymphocytes that can be coadministered with repeated vaccinations. A third feature is the immunosuppression associated with graft-versus-host (graft–host) disease and its therapies. Therefore, while vaccine strategies may be developed for autologous and allogeneic HSCT that have common elements, distinct considerations apply to the latter setting.

14.6

OPTIMIZING T CELL REGENERATION

Use of Homeostatic Cytokines As outlined, the size of T cell populations, or T cell “space,” is defined by homeostatic cytokines. Current evidence favors IL-7 as the principal homeostatic cytokine for naive CD4+ and CD8+ T cell populations, and IL-15 as the important homeostatic cytokine for memory CD8+ T cell populations. IL-2 has been identified as a homeostatic cytokine for regulatory CD4+ T cells [24]. Homeostatic cytokines affecting the peripheral, or thymus-independent, pathways of T cell regeneration have not yet been tested clinically in the setting of immunotherapy. IL-7 has now reached initial phase I trials and shows activities in humans similar to what has been described in basic and preclinical investigations (Ref. 25 and manuscript in preparation). IL-15 has yet to be tested in phase I trials, but its presence at high concentrations in the posttransplant period provides a setting for directed expansion of in vitro–derived memory/effector CD8+ T cell populations that have been adoptively transferred [26]. As a homeostatic cytokine, IL-7 is also increased in the same time period, thereby creating a favorable cytokine milieu for multiple T cell subsets. Enhancing Thymic Reconstitution Thymic regulation is poorly understood. Some strategies for enhancing thymic activity have been proposed for clinical testing including administration of growth hormone or keratinocyte growth factor (KGF), or blockade of androgen signaling. The last of these strategies has already been investigated in humans as well as in mice [27].

14.7

VACCINE STRATEGIES: TARGET ANTIGENS AND TOLERANCE

Vaccine approaches to the therapy of solid tumors have been extensively investigated for over fifteen years. Two critical components (in addition to elements of immune reconstitution) that must be considered for a cancer vaccine therapy to

224


be successful are the judicial choice of target antigen(s) and strategies addressing ways to overcome immune tolerance or boost responses to weak immunogens. There are numerous reviews summarizing the current knowledge regarding these issues [28–39], and some of these are covered in other chapters in this book. Several mechanisms of tumor-induced suppression of immune responses have been demonstrated. They include expression of IL-10 [40,41] or TGFβ [42–44], and presence of various immune suppressive tumor-related molecules [8,45–47]. As noted, it is reasonable to hypothesize that certain of these limitations may be lessened in parallel with reduction in tumor burden associated with HSCT, and that repair of the defective T cell repertoire may occur in the course of T cell regeneration in the same setting. 14.8

CONCLUSIONS

In conclusion, the combination of conventional chemoradiotherapies with immune interventions can become a therapy paradigm that can be tailored to a variety of clinical cancer settings depending on circumstances and stage of disease with the following principles: •

The combination of dose-intensive chemotherapy followed by immunedepleting chemotherapy provides a platform for subsequent immunotherapy by Lengthening the progression-free survival period, thus allowing time for more slowly acting immunotherapies such as vaccination to be effective Decreasing tumor-induced immune suppression associated with bulky disease Allowing for repeated immunizations to more effectively skew T cell reactivity toward tumor at both “early” (through antigen-driven peripheral expansion of T cells) and “late” (activation of new thymus-derived T cells) timepoints • Owing to the postimmune depletion defects and delay in immune reconstitution, an adequate immune response to vaccines may not occur unless the patient is coadministered T cells in the form of tumor antigen(s)-sensitized lymphocytes collected prechemotherapy. • The late recovery of thymic function (18–24 months), resulting in reappearance of naive T cells, may play a determinant role in the prevention of later disease progression, providing rationale for prolonged immune interventions, such as a late series of reimmunizations. This late recovery may be enhanced by cytokines. “Appropriate” immunotherapeutic strategies (vaccines and/or adoptive lymphocyte transfer) will be required to overcome the low antigenicity of some tumors and tumor-associated immune tolerance mechanisms.

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15 INTRATUMOR GENERATION OF VIGOROUS ANTITUMOR IMMUNE RESPONSES Ping Yu and Yang-Xin Fu

15.1

INTRODUCTION

Given the vast number of genetic changes associated with carcinogenesis, it would not be unexpected that tumors express many neoantigens. Lines of convincing evidence also exist to indicate that many cancers are antigenic [1]. Even unchanged germ cell proteins ectopically expressed in tumors are recognized by the T cells of cancer patients [1]. Mere recognition by adaptive immunity, however, is insufficient, as even antigenic cancers rarely regress spontaneously. An effective antitumor immune response not only depends on the presence of precursors of tumor-reactive lymphocytes in the tumor-bearing host but also requires at least three additional conditions: (1) naive tumor-specific T cells must encounter the tumor antigen in an environment of sufficient quality to permit expansion and maturation to effector cells, (2) the effector T cells must be capable of reaching the site of the malignancy, and (3) lymphocytes at the tumor site must be able to appropriately execute effector functions to destroy cancer cells. Multiple mechanisms employed by tumors to hinder the immune response at each of these steps have been identified. Cancer Vaccines and Tumor Immunity Edited by Rimas Orentas, James W. Hodge, and Bryon D. Johnson Copyright © 2008 John Wiley & Sons, Inc. 229

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INTRATUMOR GENERATION OF VIGOROUS ANTITUMOR IMMUNE RESPONSES

Overcoming tumor-associated immune-suppressive mechanisms to induce potent antitumor immunity is the goal of cancer immunotherapy.

15.2

PRIMING OF TUMOR-SPECIFIC T CELLS

Impeded by Tumor Stroma All solid tumors are composed of malignant cells that are embedded in a stroma composed of a mixed population of nonmalignant cells: bone marrow (BM)-derived macrophages, granulocytes, non-BM-derived endothelium, fibroblasts, and cells of the vasculature. Priming of tumor-specific T cells is impeded by this tumor stroma. Although still poorly understood and infrequently considered a target for immunotherapy, this nonantigenic stroma may represent an important immunologic barrier that prevents immune recognition and destruction of tumors. Early experimental evidence demonstrated that immunologic rejection is relatively easy to achieve during the first few days following tumor cell inoculation, but these same therapies become entirely powerless against established tumors entrenched in stroma [2]. Inoculations of cancer cells embedded in stroma are 10–100-fold more tumorigenic than cancer cells alone in suspension [3,4]. More recent experiments have analyzed spontaneously arising primary tumors that developed from single transformed precursor cells embedded in nonmalignant stroma. In this more physiological model, nascent primary solid tumors, even when highly antigenic in vitro, fail to stimulate the proliferation of antigen-specific T cells during the initial stages in vivo. Tumor-specific T cells are nonfunctional when they proliferated at a later stage, the reasons for which are not fully understood [5]. Tumor antigen-loaded stromal cells [6] or tumor cells themselves [3] usually transport antigen to the draining lymph nodes (DNLs), the site of immune initiation. With low levels of tumor antigen, delivery may be inefficient. Thus, cancer cells embedded in stroma may be ignored altogether [3] if antigenic molecules are not expressed at high levels by the tumor cells [6]. Other experiments also suggest that tumor stroma may act as a barrier to antigen presentation and immune recognition [7,8]. It is thus critical to penetrate and bypass the nonantigenic stroma to allow priming of tumor-specific T cells and start a strong antitumor immunity.

15.3

CURRENT STRATEGIES FOR CANCER IMMUNOTHERAPY

Active immunization and adoptive T cell transfer therapy are the main strategies used thus far for cancer immunotherapy. Both of these strategies are designed to overcome the deficiency in the priming of tumor antigen-specific T cells in the cancer-bearing hosts. Cancer vaccines rely on immunization of patients with antigenic peptides, proteins, or DNA expressed by the tumor directly or delivered by DC, virus, or another vehicle. Despite its relative simplicity and safety, vaccine

THE LESSON FROM AUTOIMMUNITY

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treatments have shown very limited success [9]. Although the generation in vivo of antitumor T cells in vaccinated patients could be demonstrated by techniques such as tetramer or ELISPOT assays [10–12], clinical responses observed from these trials were few [9]. This was consistent with the finding in murine models that the presence of even large numbers of antigen-specific T cells is insufficient to mediate tumor regression [8,13]. There are a multitude of explanations for this, including the relatively inadequate numbers or avidity of the immune cells, the inability of the tumor to recruit or activate quiescent or precursor lymphocytes, shortlived effector cells, tolerance mechanisms including the lack of costimulation, anergy, and active suppressive influences produced by the tumor or the immune system itself. These obstacles must be overcome if cancer vaccines are to be effective in mediating cancer regression. Adoptive transfer therapies, in which tumor-infiltrating lymphocytes (TILs) have to be isolated from the tumor, or T cells from patients’ peripheral blood in some cases, to be expanded in a relatively antigen-specific way for adoptive transfer [14], have shown promise in a small number of highly selected melanoma patients [15]. However, the requirement for knowledge of the antigens and the potential inability to isolate or expand T cells against tumor likely limit their application to only a minority of cancer patients in at least the near future.

15.4

THE LESSON FROM AUTOIMMUNITY

The Role of Tertiary Lymphoid Structure Although cancer cells express mutant or unique proteins that the immune system can recognize as foreign [16], malignant cells are surrounded by nonmalignant stroma to form a complex multicellular “organ” resembling self. The induction of immunity against normal, nonmutated differentiation antigens expressed by tumors resembles autoimmunity. Comparable to antitumor immunity, it is often observed in murine autoimmune models that the presence of autoreactive cells alone is not sufficient to cause tissue destruction [17]. It has been reported that the organized tertiary lymphoid structure (TLS) is necessary and sufficient to induce autoimmune destruction of pancreatic islets [18]. Indeed, de novo organization of TLS is known to precede the development of a number of human autoimmune diseases as well as in the animal models [19,20]. These observations suggest that lymphoid neogenesis within the target tissue may have a critical role in initiating and maintaining immune responses against persistent antigens. TLSs are not supplied by afferent lymph vessels and are not encapsulated, which implies that they are directly exposed to signals such as stimulating antigens and cytokines from the environment. This incomplete development of TLSs could potentially result in unrestricted access of dendritic cell (DC) lymphocytes to the TLS, favoring immune activation. Although disruption of established TLS or prevention of TLS formation may be advantageous in treating autoimmune diseases, initiation of intratumor TLS formation may facilitate the eradication of tumors. Considering the T cell repertoire may be less responsive against the self-antigens involved in autoimmunity than

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against the unique antigens involved in tumor immunity, the destruction of cancers would be further facilitated by the availability of high-affinity T lymphocytes in the presence of TLS inside the tumor.

15.5

THE INTERNAL TUMOR ENVIRONMENT

Creating Lymphoid Tissue inside the Tumor to Improve Recognition Clues to understanding the signals that lead to TLS formation come from the study of signaling pathways involved in secondary lymphoid tissue organogenesis. Studies in mutant mice and blocking experiments have identified a key requirement for TNF family members, mainly lymphotoxin α1β2 (LTα1β2), and to some extent TNF, in the development and organization of lymph nodes and spleen microarchitecture [21–24]. Binding of LTα1β2 and TNF to their respective receptors, LTβR and TNFR1, induce the expression of chemokines and adhesion molecules, which directly mediate lymphocyte migration and homing [25]. The first evidence that TLS could involve the same signaling pathways that regulate lymphoid organogenesis came from studies of transgenic mice [20], in which the ectopic expression of LTα and LTβ in pancreatic islets induced the formation of in situ TLS [26–29]. Extrapolating from this, stimulation of LTβR or TNFR expressed by tumor stromal cells might promote the formation of lymphoid-like structure inside the tumor tissues for its destruction. Systemic TNFR signaling is, of course, too toxic, as evidenced by other systemic TNF treatments [30]. Soluble LTα can signal through the TNFR, resulting in the upregulation of chemokines. To avoid systemic toxic effects, recombinant LTα has been conjugated with tumor-specific antibody to be delivered specifically to the tumor tissue [31]. Targeting of recombinant LTα to the tumor elicits an efficient immune response associated with induction of peripheral lymphoid-like tissue containing l-selectin+ naive T cells and MHC class II+ antigen-presenting cells [31]. Secondary lymphoid tissue chemokin (SLC or CCL21) is among the chemokines controlled by LTβR and TNFR signaling [25]. It is normally expressed in high endothelial venules and in T cell zones of spleen and lymph nodes and strongly attracts naive T cells and DCs [32]. The expression of SLC inside the tumor resulted in a substantial, sustained influx of T cells within the mass as well as retention of DCs at the tumor site. By recruitment of T cells and DCs, SLC in the tumor may lead to extranodal priming and inhibition of tumor growth [33]. Besides LTα1β2, LTβR is activated by another member of the TNF family, LIGHT [name derived from: homologous to l ymphotoxins, shows i nducible expression, and competes with herpes simplex virus glycoprotein D for herpesvirus entry mediator (HVEM), a receptor expressed by T lymphocytes] [34]. LIGHT is a ligand that signals through two receptors, LTβR expressed on stromal cells and HVEM expressed on T cells [34,35]. LIGHT is expressed predominantly on lymphoid tissues, especially on the surface of activated DCs and T cells. Signaling via its receptor HVEM, LIGHT acts as a strong costimulatory molecule


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for T cell activation [34,36,37]. Our data indicate that the interactions between LIGHT and LTβR restore lymphoid structures in the spleen of LTα−/− mice [38]. In vivo data demonstrate that LIGHT, signaling through two receptors, mediates a microenvironment sufficient to break immunologic tolerance to self-antigens. First, ectopic expression of LIGHT in the pancreatic islets resulted in the formation of lymphoid-like structures and was necessary and sufficient for pancreatic islet destruction [18]. In addition, sustained expression of LIGHT on T cells leads to their activation, migration into peripheral tissues, and the establishment of lymphoid-like structures in situ [38,39]. LIGHT also acts as a strong costimulatory molecule for T cell activation, possibly by binding to the HVEM on T cells [36]. Therefore, LIGHT is an ideal candidate to be delivered inside the tumor to create TLS to recruit more T cells and subsequently expand antigen-specific ones inside the tumor. In addition to providing a cross-presentation pathway, tumor-reactive T cells can be activated via a direct-presentation pathway in the presence of antigens and costimulation. Indeed, LIGHT ectopically expressed in the tumor can effectively recruit and activate naive T cells. The expression of LIGHT in the tumor environment induces an infiltration of naive T lymphocytes that correlates with an upregulation of both chemokine production and expression of adhesion molecules inside the tumor [40]. Activation and expansion of these infiltrating T cells, likely via both cross- and direct-presentation pathways, leads to the rejection of established, highly progressive tumors at local and distal sites [40]. These experiments demonstrate that introduction of the lymphoid-like structure into the tumor stroma can be highly effective in enhancing antigen recognition and may be an effective strategy for cancer immunotherapy. The ability to directly prime naive T cells within the tumor itself permits several advantages: 1. The efficiency and specificity of priming will be increased because of the higher tumor antigen load in situ relative to that collected in the draining lymphoid tissues [6]. 2. A broader repertoire of tumor-specific naive T cells is recruited to the site of tumor antigens, leading to a more comprehensive response [31,41]. It has been shown that some tumor antigens may not be efficiently cross-presented because of the antigen bias in T cell cross-priming [42]. In these instances, we have demonstrated in our experimental system using the tumor cell line Ag104Ld -LIGHT that certain antigens are presented to and activate naive T cells within the tumor via a direct-presentation mechanism [42]. The same tumor would have been otherwise missed by the host immune response had it relied on solely draining lymphoid tissues and cross-presentation. 3. No additional migration steps are required for CTL to reach the site of effector function, which leads to the appearance of activated tumor-reactive T cells in a short period of time. 4. T cell responses may react more readily to the shifting tumor antigen expression profile in situ.

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T cell stimulation in the absence of costimulation can induce anergy and apoptosis of antigen-specific T cells [43–45]. Costimulation has also been shown to greatly enhance tumor-specific T cell function during the effector phase [46]. Earlier studies show that LIGHT provides CD28-independent costimulation [36], which may be essential for the selective and effective activation, expansion, and maintenance of tumor-specific T cells among infiltrating naive T cells in Ag104Ld -LIGHT tumors. It appears that the introduction of lymphoid-like milieus, such as the combination of chemokines, adhesion molecules, and costimulatory property, inside tumor tissues may allow LIGHT to become a potent molecule for better immune response against established tumor. The Immunosuppressive Environment inside Tumors Even with immune recognition of cancer, which can occur in cancer patients and is evidenced by the frequent observation of T cell infiltration into cancerous tissues [47–50], it is rare for such tumor-infiltrating T cells to induce the spontaneous rejection of established tumors. Accumulating evidence indicates that the tumor environment contains cells and cytokines that actively suppress primed effector T cells [51,52]. High concentrations of transforming growth factor β1 (TGFβ1) are frequently found in solid tumors and production of TGFβ1 by cancer cells or tumor stromal cells interferes with effective immune rejection of transplanted solid tumor fragments [53]. TGFβ1 is a cytokine essential for the generation and survival of CD4+ CD25+ regulatory T cells [54,55], which may themselves produce TGFβ1 and IL-10 to reinforce the immune suppressive loop that exists in the tumor environment. While active TGFβ1 does not inhibit lysis by CTL, it can inhibit maturation of T cells to that effector state [56]. This explains, at least in part, why CD8+ T cells resistant to TGFβ1 can mediate tumor rejection [57]. Highly antigenic tumors may yet fail to regress in the host owing to an accumulation of regulatory T cells (Tregs) within the tumor microenvironment [58]. These CD4+ CD25+ Tregs seem not only to inhibit developing tumor-specific CD8+ T cells from gaining full effector function, but also can even block transferred, fully activated tumor-specific T cells from mediating tumor rejection in vivo [59]. There is little question that tumors can develop, establish, and persist while retaining their full antigenicity in normal hosts [5]. Therefore, neutralizing anti-TGFβ1, anti-CD4, or anti-IL-10 antibodies injected into established solid tumors may be effective in counteracting the immunosuppressive environment that prevents tumor-specific T cells from acquiring full effector function [58]. We have demonstrated that the local intratumoral depletion of these Tregs changes the cytokine milieu of the tumor, unmasks the immunogenicity of tumor, and reverses CTL tolerization, leading to the rapid rejection of well-established tumors [58]. Our data support the idea that Tregs inhibit not only the early priming events but also the effector function of T cells inside tumors, which has been shown by a more recent study [59]. Established progressive tumors cause splenomegaly and increased Gr1+ CD11b+ immature myeloid cells [60,61]. These cells can promote angiogenesis [62] and stimulate growth of cancer cells in vitro [63]; they also have powerful


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immunosuppressive effects in vitro. Splenic Gr1+ immature myeloid cells differentiate into suppressive tumor-associated macrophages (TAMs) [64,65]. Treatment of mice with anti-Gr1 suppresses tumor growth in T-cell-deficient mice [66] and allows normal mice to reject otherwise lethal tumor challenge [67]. This treatment may reduce Gr1+ splenic precursors of TAM, abolish a neutrophil-dependent macrophage recruitment mechanism in the tumor by eliminating Gr1+ granulocytes [68], and/or eliminate Gr1+ tumor growth-promoting cells within the tumor microenvironment. Again, it is not clear whether myeloid suppressor cells or TAM would interfere with tumor rejection In vivo when fully activated tumor-specific T cells are transferred. Changing the Tumor Environment In addition to stromal cells and T cells, LIGHT also interacts with other cells, such as natural killer (NK) cells and dendritic cells (DCs) [69,70]. LIGHT can activate NK cells via HVEM, which produce substantial amounts of IFNγ, leading to further activation of effector CD8+ T cells inside the tumor [69]. LIGHT also stimulates DCs and promotes their expansion and/or survival [70]. This may lead to more efficient priming of T cells. In addition to costimulation of T cells, LIGHT alters the phenotype of macrophages to one producing an abundance of IFNγ, possibly through an indirect pathway (unpublished data). The expression of LIGHT inside the tumor switches the environment to a proinflammatory one [40], which seems to facilitate further accumulation of effector T cells and tumor rejection. These results are consistent with more recent studies using CpG oligodeoxynucleotides [71], which suggest that the induction of a proinflammatory microenvironment in autochthonous tumors enhances extravasation of effector cells and cancer destruction. It remains to be determined whether and how a LIGHT-conditioned microenvironment can also prevent in situ activated T cells from succumbing to tumor-mediated immunosuppression. Generation of CTL in the LIGHT-Mediated Tumor Environment for Treatment of Metastases Micrometastases can establish early in heterogeneous primary tumor development and seed distal sites prior to clinical detection [72]. Therefore, at the time of diagnosis many cancer patients already have microscopic metastases, an observation that has led to the development of postsurgical adjuvant therapy for patients with solid tumors. Despite advances in early detection and modifications to treatment regimens, success has been limited and optimal treatment of metastatic disease continues to pose a major challenge in cancer therapy. We have shown that expression of LIGHT in the tumor can lead to rejection not only at a local site but also at the distal sites. To develop more clinical relevant approaches, adenovirus vectors that express LIGHT (Ad-LIGHT) have been constructed to deliver LIGHT into the tumor tissue. The advantages of adenovirus are (1) high production of nonreplicable virus, (2) easy manufacture, (3) activation of

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innate immunity, (4) an ability to express its carrying gene in undividing cells, and (5) ready expression in most tumor cell lines. Administration of Ad-LIGHT into the tumor tissue leads to the complete rejection of an aggressive fibrosacoma Ag104Ld and retards the growth of other tumors, such as melanoma B16, colon cancer MC38, and breast cancer 4 T1 in mice [76]. The poorly immunogenic 4 T1 mammary carcinoma closely mimics human breast cancer in its anatomic site, immunogenicity, growth characteristics, and more importantly, metastatic properties [73–75]. We have demonstrated in this model that generating immune responses in primary tumor tissues prior to surgical resection can produce tumor-specific effector T cells sufficient to eradicate distant metastases in a CD8-dependent fashion. Local treatment with Ad-LIGHT initiated priming of tumor-specific CD8+ T cells directly in the primary tumor, with subsequent exit of CTL, which homed to distal tumors to elicit immune-mediated eradication of spontaneous metastases [76]. Conventional treatment such as surgical removal of tumor followed by radiation and chemotherapy may prevent effective immune recognition of cancers due to the loss of a major source of antigens, and damage to preexisting CTL by radiation and chemotherapy. An alternative strategy would be to utilize the primary tumor as the site of CTL priming prior to surgical resection. Delivery of LIGHT expression within the tumor environment recruits naive T cells and generates tumor-specific CTLs that can survive and exit the microenvironment to patrol peripheral tissues and eradicate disseminated metastases (see Fig. 15.1 for model).

Tumor environment HVEM LIGHT

NK

SLC LTβR

Recruitment

SLC HVEM

LTβR Stromal cells

DC

Expansion

Activation IFN-γ

Recruitment Naïve T Naïve T

Chemokines and adhesion molecules Activation and expansion

Effector T

Distal tumors (metastases)

Figure 15.1. Tumor environment.

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15.6

237

CONCLUSIONS

Understanding the balance of antitumor effectors versus suppressors may be important in determining the outcome of immune responses inside tumors. Our more recent studies have demonstrated that creation of lymph-like structures within the tumor leads to rapid recruitment of naive lymphocytes and expansion of CD8+ T cells. This may be a means of generating a predominantly proinflammatory environment, resulting in the rejection of both primary tumor and metastases [40] (Fig. 15.1). We further demonstrated that the depletion of regulatory T cells inside the tumor is an efficient means of converting the antiinflammatory environment inside tumor to a proinflammatory one [58]. The ideal combination treatment would more rapidly expand the effector cells at the tumor site, while blocking local suppressive factors, including depletion of regulatory cells. This could prove a potent strategy for enhancing antitumor immunity and permitting a clinically desirable outcome for cancer patients. Therefore, by overcoming the suppressive mechanisms in the cancer microenvironment, tumor tissue might be used to generate a stronger immunity against itself. It is likely that such immunotherapy, followed by surgical removal of primary tumor or local radiation, which increases antigen load to improve cross-priming of T cells, allows CTL, previously generated inside tumor to eradicate residual or undetected metastatic tumor cells. How to design immunotherapy and avoid immune suppression induced by chemotherapy and radiation might be the key to reduce metastases in patients.

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16 CANCER IMMUNOTHERAPY: UNTAPPING THE POTENTIAL OF COSTIMULATORY MOLECULES BEYOND CTLA-4 Mathew Augustine and Lieping Chen

16.1

INTRODUCTION

Over 200 years have passed since Edward Jenner ushered the world into the field of immunology and immunoprophylaxis by treating healthy individuals with cowpox and observing the protection that his strategy of vaccination provided against the fatal human disease smallpox. Later, William Coley reported his cogent observation of tumor regression in patients harboring microbial organisms within malignant tissue. Coley’s observation, along with Paul Ehrlich’s theory on the role of the immune system in controlling tumor growth, ignited a contentious debate that has raged since the early 1900s on the nature of tumor cell “foreignness” and the immune system’s capacity to recognize and eradicate neoplastic tissue. Naturally, out of these ideas evolved the concept of vaccination as an immunotherapeutic strategy to eradicate cancer. Studies performed in the mid-1950s revealed for the first time that the immune system recognizes tumor-associated antigens and indicated that the immune system could, on some level, sense tumor as nonself [1].

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More recent experiments have not only validated Ehrlich’s hypothesis, but have implicated all levels and specific components of the immune system (both innate and adaptive) in influencing antitumor immune responses [2]. However, clinically detectable cancers have invariably either escaped tumor recognition altogether or devised mechanisms to outwit their opponent [3], ultimately rendering the immune system impotent in tumor control. In T cell signaling, at least two signals are considered integral in defining the fate of naive T cells and their competence to combat disease. Major histocompatibility complex (MHC)–peptide interactions with T cell receptor (TCR) provide the first signal in immune responses. However, while the MHC–peptide interaction with TCR is essential in determining specificity and initiating T cell activation, this process is not sufficient to determine the fate of T cells [4]. Costimulatory and coinhibitory molecules, which are cell surface glycoprotein molecules, supply additional signal or “second signal” to T cells through antigen-presenting cells (APCs), providing context to the state of activation or inactivation that develops within the T cell [5]. The second signal delivered is most likely a product of both costimulatory and coinhibitory influences. Positive signals result in T cell proliferation, expansion, and survival; specific cytokine release; and cytotoxic effector responses. Negative signaling results in T cell death, reduced memory responses, suppressor T cell induction, and immunosuppressive cytokine secretion. An example of a signaling system with both positive and negative output is the B7/CD28/CTLA4 pathway, the most thoroughly studied cosignaling pathway [6,7]. CD28 ligation on T cells results in costimulatory signals delivered by B7-1 (CD80) and B7-2 (CD86). CTLA4 provides coinhibitory signals also through interactions with B7-1 and B7-2. CTLA4-deficient mice exhibit severe lympoproliferative disease and autoimmunity [8,9]. Considering the range of immune responses that can be elicited by these molecules, it is not surprising that tumor cells have enlisted costimulatory/coinhibitory molecule to help in their armamentarium to render them poorly immunogenic or nonimmunogenic. The influence of costimulatory/coinhibitory molecules on the state of T cell activation and their expression on cancer cells has made these molecules attractive agents for immunotherapy. While anti-CTLA4 antibody therapy [10] is the first in this class of tumor immunotherapy to reach clinical trials and exhibit efficacy, its association with the development of grade 3/4 autoimmunity supports investigation into the therapeutic potential of other costimulatory/coinhibitory molecules, as they may grade the potency of the immune response on a different contextual scale. The molecules that are currently being brought to phase I clinical trials for therapeutic applicability are members of the PD-1/B7-H1/B7-DC pathway and the CD137/CD137L pathway. These molecules are discussed in this chapter, detailing the fundamental studies that have illuminated their potent influence on the state of immune tolerance and activation, with a focus on manipulation of this pathway as an immunotherapeutic strategy toward tumor clearance.

PD-1/ B7-H1/ B7-DC PATHWAY

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Programmed death 1 (PD-1), a member of the immunoglobulin gene superfamily, was initially identified by subtraction hybridization in an attempt to discover genes expressed during classical cell death progression in two cell types, a murine T cell hybridoma (2B4.11) stimulated with PMA and ionomycin, and interleukin 3 (IL-3)-deprived LyD9, a lymphoid/myeloid progenitor cell line [11]. Initially, PD-1 mRNA expression was identified in the thymus and was increased with enhancement of in vivo thymic apoptosis by anti-CD3 antibody. Despite the apparent connection between cell death and expression of PD-1, clear evidence to support the role of PD-1 in apoptosis was unavailable, as LyD9 cells transfected with PD-1 mRNA showed neither increased apoptosis nor defects in proliferation compared to the parental line. Instead, subsequent studies provided evidence that activation of the Jurkat T cell leukemia line as well as peripheral blood mononuclear cells (PBMCs) resulted in the upregulation of human PD-1, and that this expression did not correlate with apoptosis [12]. PD-1 expression was also identified on thymic, splenic, and lymph node T cells after either anti-CD3 or concanavalin A (ConA) treatment as well as on B cells after in vitro stimulation with anti-IgM antibody [13]. Similar to earlier studies, cell deprivation did not result in increased PD-1 expression. These results offer further evidence against a role for PD-1 in apoptosis and supported a possible link between activation of T and B lymphocytes and PD-1 expression. The genes PD-1, CD28, CTLA4, and ICOS, are member of the same costimulatory family and share approximately 20% sequence identity. The mature form of murine PD-1 consists of 268 amino acids and is a type I transmembrane glycoprotein exhibiting 60% sequence identity to its human counterpart [14]. PD-1, in contrast to CD28, CTLA4, and ICOS, does not contain interchain disulfides known to mediate covalent dimerization and was found to be a stable monomer both in solution and on the cell surface [15]. Notably, PD-1 contains two tyrosine residues in its cytoplasmic domain. One is an immunoreceptor tyrosine–based inhibiting motif (ITIM) that is generally associated with inhibiting immune responses [16]. The second has been identified as an immunoreceptor tyrosine-based switch motif (ITSM) and is thought to be the actual domain through which inhibitory signals are relayed by PD-1 ligation [17]. The inhibitory activity was initially proposed to result from association of SHP-2, a cytoplasmic tyrosine phosphatase, with the ITIM [18]. Further experiments, however, demonstrate that the ITSM, and not the ITIM, mediates the inhibitory effects of PD-1 as selective mutations of the ITSM resulted in the inability of PD-1 to abrogate T cell activation [19]. The PD-1 gene is known to associate with two ligands, B7-H1 (designated as PD-L1) and B7-DC (also designated as PD-L2). B7-H1, a member of the immunoglobulin superfamily, is a type I transmembrane protein consisting of 290 amino acids and was identified through a homology search of a human cDNA

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expressed sequence tag (EST) database using the sequences for B7-1 and B7-2 [20]. The mRNA transcript for B7-H1 was identified in the majority of tissues including heart, skeletal muscle, placenta, lung, thymus, spleen, kidney, and liver tissues. In contrast to this broad distribution of mRNA, antibody against the B7-H1 protein revealed its constitutive expression on limited tissues/cells and the expression is associated largely with tissue macrophages. However, surface expression of B7-H1 could be detected in virtually all the cells tested so far on induction with IFNγ [21,22]. B7-DC, a 247-amino-acid member of the immunoglobulin superfamily, was identified as a second ligand that binds PD-1 [23,24]. It was initially discovered by its differential expression between dendritic cells (DCs) and macrophages. However, expression of B7-DC on macrophages was subsequently reported after incubation with GM-CSF, IL-4, and IFNγ [25]. With accumulating evidence confirming the expression of PD-1 on activated T and B cells, and the constitution of its cytoplasmic domain with both an ITIM and ITSM, knockout studies were performed to identify potential phenotypes indicative of immune dysregulation. PD-1-deficient mice were produced in two different backgrounds and presented with distinct forms of autoimmunity. PD-1-deficient C57BL/6 (B6) mice spontaneously developed lupus-like glomerulonephritis and progressive arthritic lesions along with deposition of IgG3 and C3 complement protein within the renal glomeruli. This phenotype presented earlier and with greater severity when the Fas gene mutant, lpr, was introduced [26]. Noticeably, PD-1 gene disruption in Balb/c mice resulted in the development of severe dilated cardiomyopathy with subsequent premature death as a result of congestive heart failure [27]. This phenotype did not develop in PD-1, recombinase activating gene (RAG) double-knockout mice, indicating that the effect was mediated through T and B cells. The sera of the affected mice contained high titers of autoantibody against a heart-specific protein, which was subsequently identified as cardiac troponin I [28]. Further supporting an association between autoimmunity and PD-1, human genetic studies showed an association between the presence of type 1 diabetes and systemic lupus erythematosus with single-nucleotide polymorphisms (SNPs) in the intronic region of the PD-1 gene [29,30]. Therefore, these findings support a general role for signaling through PD-1 via PD-1 ligands in controlling T and B cell responses, with disruption of this pathway resulting in distinct autoimmune outcomes in different mouse strains. The role of PD-1 in moderating autoimmune responses was further exemplified by utilizing anti-PD-1, anti-B7-H1, and anti-B7-DC antibodies in a murine model of experimental autoimmune encephalitis (EAE) [31]. Blockade of PD-1 accelerated the onset of disease, induced greater EAE severity score, increased anti-MOG antibodies, and increased IFNγ-secreting T cells. Surprisingly, while expression of both PD-1 and B7-H1 (but not B7-DC) on spinal cord sections of mice with EAE was exhibited, blockade of B7-DC, but not B7-H1, resulted in more severe disease. While the aforementioned studies established a mediating role for PD-1 in inhibiting autoimmune pathogenesis, the mechanism by which the PD-1 ligands signal through PD-1 has been less clear. Molecular modeling of B7-H1 and B7-DC indicates significant structural heterogeneity at the interface where the two


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molecules interact with PD-1, supporting unique binding features of the two ligands [32]. In addition, ligand mutants could still perform costimulatory functions with increased cytokine production in vitro, supporting previous experiments indicating a stimulatory role for B7-H1 binding. Interestingly, these studies also suggest the possible existence of an unknown second receptor that delivers stimulatory signals distinct from PD-1. Early studies on the function of B7-H1 provided conflicting results supporting both costimulatory and coinhibitory properties. B7-H1 was shown to promote in vitro proliferation of T cells as well as production of IL-10, IFNγ, and GM-CSF in the presence of suboptimal concentrations of anti-CD3 through the use of a B7-H1:Ig fusion protein construct. Both processes were shown to require IL-2. In addition, B7-H1 was shown to augment T cell proliferation in the absence of CD28, but was unable to generate cytolytic T cell responses after transfection of the P815 tumor line [33]. Supporting a negative role, B7-H1 was identified on monocytes and dendritic cells and was capable of inhibiting proliferation of T cells through PD-1 as well as the cytokine production of IFNγ and IL-10 [34]. B7-H1 was also shown to inhibit proliferation and IL-2 production with reversal in the presence of soluble agonistic anti-CD28 antibody [35]. However, over prolonged activation, B7-H1:PD-1 inhibition predominates by decreasing IL-2 production through CD4+ T cells, which partially influences CD8+ T cells because of their intrinsic inability to secrete IL-2 on activation. Furthermore, a study utilizing a blocking antibody against B7-H1 on DCs could reverse anergized T cells [36]. These latter results support a model in which B7-H1 modulates the immune response by tuning down activation and proliferation of T cells, rendering them unresponsive. Much like B7-H1, B7-DC has exhibited a role in both positive and negative signaling. B7-DC was reported to costimulate T cell proliferation and increase IFNγ production from naive T cells [24]. Crosslinking B7-DC with antibody resulted in increased MHC:peptide presentation, IL-12 production, promoted DC survival, improved DC migration to draining lymph nodes, and induced antigen-specific T cell immune responses [37,38]. A subsequent study reported a role for B7-DC in inhibition of T cell proliferation, CD4+ T cell cytokine production, and B7/CD28 signaling by the PD-1:B7-DC pathway [23]. A B7-DC knockout mouse was generated and provided both in vivo and in vitro evidence of a role for B7-DC in tuning Th1 responses as knockout mice displayed reduced CD4+ T cell IFNγ production and IFNγ-dependent IgG2a humoral responses [39]. CD8+ T cell responses and CTL activity were also reduced. Related to this finding was evidence of accelerated hepatic tumor growth in the knockout mice compared to wild-type mice. These results indicate an important role for B7-DC in fine-tuning the adaptive immune response. In support of an inhibitory role, monoclonal antibodies against B7-DC on dendritic cells resulted in increased T cell proliferation and increased cytokine production of IFNγ and IL-10 [40]. Recently, a second study utilizing a B7-DC knockout mouse was described [41]. CD4+ T cells, in the presence of knockout APCs, were more sensitive to anti-CD3 antibody stimulation, even in the absence of CD28, and produced higher IFNγ and IL-4 levels. CD8+ T cells also displayed

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higher proliferation and increased IFNγ and TNFα secretion. Spleen cells recovered from mice immunized with ovalbumin and CFA and stimulated in vitro with peptide resulted in enhanced proliferation and increased Th1 and Th2 cytokine production. Furthermore, B7-DC knockout mice failed to induce oral tolerance. These results support a role for B7-DC in negative regulation of T cells and maintenance of oral tolerance. Adding to the complex relationship between peripheral B7-H1 expression and tolerance or activation, allografts of pancreatic islet β cells from transgenic mice bearing B7-H1 exhibited accelerated rejection [42]. The transgenic mice showed evidence of elevated glucose levels and periinsulitis that was not seen within the wild-type control group. The presence of B7-H1 increased CD8+ T cell proliferation and transfer of antigen-specific T cells with concomitant administration of B7-H1:Ig fusion protein resulted in increased cell division. B7-H1 was also found to play an important role in regulating a murine form of colitis [43]. Colonic samples taken from SCID mice transferred with CD4+ CD45RBhi T cells were found to express high levels of PD-1, B7-H1, and B7-DC. When given anti-B7-H1 antibody, these same mice showed no evidence of overt experimental colitis and had reduced IL-2 and Th1 cytokine production (IFNγ and TNFα). These results provide further support toward the potential involvement of a second receptor for B7-H1-positive signaling. The possibility of “reverse signaling” on T cells through B7-H1 was also described, as rheumatoid arthritis patients possess autoantibodies to B7-H1, and the presence of this autoantibody correlates with active status of disease [44]. On activation, human CD4+ T cells were shown to augment the expression of B7-H1. Antibody to B7-H1 was shown to enhance the proliferation of CD4+ T cells in vitro in the presence of suboptimal concentrations of anti-CD3 antibody, although the effect was less than that produced by anti-CD28 antibody. These studies support a role for B7-H1 in both forward and reverse signaling involving T cell activation and autoimmune pathogenesis, although the number and nature of target receptors are undefined. Despite evidence suggesting a stimulatory role for B7-H1 in both forward and reverse signaling, data have accumulated supporting a predominant role for this molecule in mediating inhibitory immune signals and peripheral tolerance through binding to PD-1. Female NOD mice administered anti-PD-1 or anti-B7-H1 antibodies exhibited rapid development of diabetes with worse insulitis scores [45]. B7-H1 was found expressed on the islets of NOD mice that developed diabetes. The same effect could not be elicited with anti-B7-DC antibodies. Interestingly, mice given anti-CTLA4 antibodies developed disease only when the antibody was given as a neonate, while anti-PD-1 and anti-B7-H1 antibodies could induce disease regardless of age, suggesting that CTLA4 mediates its effects at an early stage of development, while PD-1:B7-H1 influences progression of disease at all developmental stages. This may be related to the presence of B7-H1 within the peripheral tissue, where MHC class I is expressed, supporting an important role for this molecule in maintaining peripheral tolerance to autoreactive CD8+ T cells. In a recent set of experiments, B7-H1/B7-DC (PD-L) double-knockout mice were crossed onto the NOD mouse and exhibited clear evidence of an earlier onset


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of autoimmune diabetes with complete penetrance [46]. The transfer of splenocytes from prediabetic WT NOD mice into WT or PD-L-deficient NOD SCID mice resulted in earlier autoimmune diabetic progression in the PD-L knockout mice, indicating that the presence of these molecules on nonlymphoid cells suppresses autoimmune progression. Furthermore, the presence of PD-L on hematopoietic cells was not sufficient to prevent diabetes. Finally, B7-H1 and not B7-DC was shown to influence autoimmune progression. These studies in combination suggest that B7-H1 expression on pancreatic β-islet cells is important in delivering inhibitory signals that maintain peripheral tolerance and prevent autoimmunity. To show that B7-H1 exhibits an important negative regulatory role in vivo, mice lacking B7-H1 were generated [47]. B7-H1 knockout CD4+ and CD8+ T cells, on stimulation, produced elevated levels of IFNγ compared to WT controls. For CD4+ T cells, this occurred at suboptimal anti-CD3 antibody concentrations, while for CD8+ T cells, this occurred at higher anti-CD3 concentrations. This effect could be delivered by APCs as B7-H1-deficient dendritic cells showed augmentation of IFNγ secretion by CD4+ T cells. B7-H1 knockout mice developed worse EAE and increased IL-2 and IFNγ. Human endothelial cells exposed to IFNγ could also be induced to express B7-H1 [21,22]. Blocking the B7-H1:PD-1 interaction enhanced IL-2 and IFNγ expression. These results support a negative role for B7-H1 on both DC and endothelial cells in T cell activation and effector function, thereby supporting its role in the induction of tolerogenic states. The augmented expression of PD-1 on activated T cells suggests an important role for PD-1 during a distinct phase of the T cell response. In addition, the relationship between PD-1 and autoimmune disease implicates PD-1 and its ligands in mechanisms related to peripheral tolerance. Peripheral tolerance is a critical aspect of normal liver immunity exemplified by the fact that allogeneic liver transplantation can occur without the utilization of immunosuppressive regimens as well as antigen-specific tolerance induction by antigen fed through the portal vein. With consideration toward these associations, B7-H1 was identified on nonparenchymal cells within the liver, namely, sinusoidal endothelial cells and Kupffer cells [48]. Coculture of PD-1-deficient T cells with liver nonparenchymal cells augmented their proliferative responses and had prolonged IFNγ production compared to wild-type T cells. The use of an anti-B7-H1 mAb in this same system resulted in increased proliferation comparable to that of PD-1-deficient T cells. These results were consistent whether Kupffer cells or liver sinusoidal endothelial cells were used. Moreover, PD-1-deficient mice infected with adenovirus exhibited higher numbers of proliferating CD4+ and CD8+ T cells in the liver as compared to WT mice. The inflammation was more severe over a shorter timecourse, resulting in rapid clearance of the virus from the liver. These results support an important role for the PD-1:B7-H1 pathway in augmenting hepatic immune tolerance. Subsequently, B7-H1-deficient mice were generated and shown to have increased selective accumulation of antigen-specific CD8+ T cells in the liver [49]. Apoptosis, not enhanced cell proliferation, was identified as the primary cause of this accumulation. Furthermore, experimental autoimmune hepatitis caused by elevated CD8+ T cells could be rapidly induced in these mice. T cell responses could also

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be inhibited by activated hepatic stellate cells on B7-H1 induction [50]. These stellate cells induced a state of hyporesponsiveness in allogenic or anti-CD3-induced T cell activation, attributed to enhanced apoptosis and not decreased cytokine production. These results provide further evidence of a critical role for B7-H1/PD-L1 on various nonparenchymal cells within the liver in deleting activated intrahepatic CD8+ T cells and furthering its claim as a mediator of peripheral tolerance. Previously, chronic infection with LCMV was shown to result in the development of specific “exhausted T cells” with reduced cytokine production, proliferation, and capacity to lyse target cells [51,52]. In an attempt to identify genes in “exhausted T cells” that might be preferentially upregulated during chronic viral infection, PD-1 was culled from a genomewide microarray analysis [53]. Fluorescence-activated cell sorter (FACS) analysis confirmed these results as chronic LCMV infection resulted in antigen-specific, PD-1+ , CD8+ T cells. Persistently infected mice that received anti-B7-H1 blocking antibody exhibited higher antigen-specific CD8+ T cells expressing IFNγ and TNFα on peptide stimulation and cleared virus more effectively from multiple anatomic locations than did control mice. Removal of CD4+ T cells, rendering “exhausted” CD8+ T cells “helpless,” did not alter this outcome as these mice displayed higher antigen-specific CD8+ T cells in multiple compartments, increased proliferation, contained more IFNγ-producing cells, and functionally exhibited more effective specific lysis and viral clearance. Importantly, these “exhausted” cells persisted for several weeks after blockade. This result was due to reactivation of PD-1+ cells on blocking the B7-H1 pathway rather than generation of new responses from naive T cells. Three papers extended these results by examining the role of the PD-1 pathway in HIV infection [54–56]. Utilizing MHC class I tetramers associated with common HIV epitopes, PD-1 expression, and not CTLA4, was found to be highly expressed on CD8+ T cells from chronically infected HIV patients in comparison to seronegative controls. An inverse correlation was observed between the percentage of proliferating cells and expression of PD-1. Moreover, PD-1 expression correlated with viral load and inversely correlated with CD4+ T cell count, indicating an association between PD-1 expression, antigenic load, and disease progression. The use of antiretroviral treatment coincided with a decrease in PD-1 expression on CD8+ T cells. Inhibition of the PD-1:B7-H1 pathway with blocking antibody resulted in the expansion of tetramer-positive cells with increased IFNγ, TNFα, and lymphotoxin-α expression. PD-1 expression was once again associated with influencing apoptosis. These intriguing results further illuminate a critical role for PD-1:B7-H1 interactions in influencing T cell function and the role that directed therapy against this pathway might play in the rejuvenation of “exhausted” CD8+ T cells in persistently infected hosts with chronic viral infection or malignant disease. The administration of antiretroviral therapy suggests an important relationship between antigen level and PD-1 expression. Most tantalizing is the potent response that can be rescued in mice lacking CD4+ T cell help as these results have potential applicability in disease states such as HIV infection, where unresponsiveness or progressive depletion of CD4+ T cells occurs.


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Considering the influence of the PD-1, B7-H1, and B7-DC pathways in modulating immune responses, cancer cells were examined to identify both expression and functional relevance of B7-H1 and B7-DC on the tumor cell surface. As for B7-DC, one report indicated that primary mediastinal B cell lymphoma could be discriminated from diffuse large B cell lymphoma by its expression. B7-DC was also identified on Hodgkin’s lymphoma cells [57]. While most normal solid tissues lack B7-H1 expression, FACS analysis of tumor cell lines, including lung and ovary, showed surface expression of this molecule [58]. On treatment with IFNγ, lung, colon, ovary, duodenum, leukemia, lymphoma, placenta, and melanoma cancer cell lines also upregulated expression of B7-H1. A CD8+ T cell clone generated against a melanoma cell line transfected with B7-H1 resulted in a 62% increase in apoptosis compared to controls. Antibody against B7-H1 or the delivery of PD-1:Ig fusion protein inhibited apoptosis and increased the number of viable T cells. A similar result was also found using a breast cancer cell line that constitutively expresses B7-H1. Interestingly, these CTLs lack PD-1 expression, and this finding further supports the possibility of a non-PD-1 receptor mediating this apoptotic effect. Furthermore, in vivo delivery of antigen-specific CD8+ T cells into RAG−/− mice with B7-H1-transfected P815 cells was compared to transfer of CD8+ T cells into RAG−/− mice with mock-transfected P815 cells. T cells from the mice that harbored B7-H1-transfected P815 tumor cells showed decreased numbers and underwent apoptosis while blockade of B7-H1 resulted in inhibition of growth of the B7-H1-transfected P815 cells. These results indicate that B7-H1 functions in inducing apoptosis of activated T cells as a mechanism for immune evasion. This study was supported by a subsequent report linking B7-H1 to tumor immune evasion by using a similar model system. P815 cells transfected with B7-H1 were more resistant to CD8+ T cell killing than were nontransfected cells, with abrogation of this effect on exposure to anti-B7-H1 antibody [59]. Subcutaneous injection of tumor cells expressing B7-H1 into syngeneic hosts exhibited progressive local tumor growth and systemic tumor invasion, and succumbed to malignancy earlier compared to mice inoculated with nontransfected P815 cells. These events were partially reversed in mice receiving anti-B7-H1 antibody. Furthermore, inoculation of tumor cells expressing B7-H1 into PD-1 knockout mice in comparison to PD-1+ mice resulted in tumor clearance in the knockout mice, suggesting that signals delivered through the B7-H1:PD-1 pathway confer on tumor cells a mechanism to evade immune responses and that blocking this pathway could be a rational immunotherapeutic strategy. In a study of human glioma cell lines, cell surface expression of B7-H1, but not B7-1 and B7-2, was identified and increased in response to IFNγ exposure [60]. Coculture of glioma cells with CD4+ or CD8+ T cells and anti-B7-H1 antibody resulted in increased IFNγ and IL-2 cytokine secretion as well as increased expression of the T cell activation marker CD69. Similarly, blockade by anti-B7-H1 antibody of a B7-H1-transfected squamous cell carcinoma cell line enhanced adoptive T cell immunotherapy and improved survival when tumor cells were injected in vivo [61]. These studies provide strong supportive evidence of the role of B7-H1 in

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the induction and maintenance of immune tolerant states, and more importantly, the utilization of this molecule by tumor cells to evade antitumor immune responses. The pervasiveness of B7-H1 expression within the tumor microenvironment was illustrated by the expression of B7-H1 on monocyte-derived myeloid dendritic cells (MDCs) within draining lymph nodes from ovarian cancer as well as within the tumor itself [62]. MDCs, exposed to tumor macrophages and IL-10, which upregulate B7-H1, exhibited impaired capacity to stimulate T cell proliferation. Similar to other studies, the use of antibody against B7-H1 resulted in increased T cell proliferation and increased IFNγ and IL-2 production with decreased IL-10 production. Tumor-bearing mice, transferred with MDC-activated T cells in the presence of anti-B7-H1 antibody, showed impaired tumor growth over a defined time period as compared to untreated mice. Increased IFNγ-expressing T cells were recovered from the tumor treated with anti-B7-H1 antibody; however, the numbers of CD8+ T cells and apoptotic T cells were equivalent, suggesting that the effect on tumor control regulated by B7-H1 blockade in this study was related to a qualitative shift in IFNγ-expressing T cells. With the reports of B7-DC expression limited to a small subset of lymphomas, more recent experiments have shed light on the potential role for B7-DC modulation in immunotherapy. As was previously mentioned, a crosslinking antibody against B7-DC was shown to induce IL-12 production and enhance T cell responses [37,38]. Concomitant transfer of this anti-B7-DC antibody with transfer of a poorly immunogenic melanoma cell line to mice rendered a significant number of these mice tumor-free [63]. Furthermore, this effect appeared to be mediated through dendritic cells exhibiting enhanced pinocytotic function without increased expression of B7-1 or B7-2 both in vitro and in vivo. Both CD4+ and CD8+ T cells, but not NK cells, were necessary for the antitumor effect seen by crosslinking B7-DC in vivo and were resistant to secondary challenges of tumor cells. This response was shown to be tumor-specific as secondary challenge with unrelated tumor did not provide a protective effect. The results of this study support a role for B7-DC in activation of dendritic cells, acquisition of antigen for presentation to CD4+ and CD8+ T cells, enhancement of CTL responses, and subsequent antitumor immunity that is potent and selective. The role of B7-DC on tumor cells was assessed by transfecting J558 plasmacytoma cells with B7-DC [64]. Mice given B7-DC-transfected J558 tumor cells showed evidence of immediate tumor growth, but then complete regression. The response was mediated by CD8+ T cells as depleting anti-CD8 antibodies, but not anti-CD4 antibodies, resulted in persistent tumor growth. Mice that received the B7-DC-transfected J558 tumor cells cleared tumor and exhibited memory response as rechallenge with tumor 2 months after primary inoculation resulted in tumor clearance. Mice given B7-DC-transfected J558 cells also elaborated production of antigen-specific CD8+ T cells that exhibited faster division. Adoptive transfer of antigen-specific T cells into mice containing B7-DC-transfected J558 tumor cells presented with higher numbers of these T cells one month after transfer as compared to mock-transfected J558 tumor cells. Interestingly, these studies also raised the possibility of a second receptor for B7-DC that might facilitate costimulation

CD137 AND CD137 LIGAND

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as a B7-DC:Ig fusion protein bound both PD-1-positive and PD-1-negative T cells. Both PD-1+ - and PD-1-deficient T cells could lyse B7-DC-transfected tumor cells. Furthermore, a blocking antibody to B7-DC reduced PD-1:B7-DC interaction, while the same blocking antibody did not prevent B7-DC binding to PD-1-negative cells, raising the possibility that B7-DC uses a different epitope to bind PD-1 positive and negative cells. These results support long-term antitumor immune responses enhanced by expression of B7-DC with T cell killing effect mediated by a PD-1 and/or a non-PD-1 receptor. The abovementioned studies provide provocative insight into the role that the PD-1:B7-H1 pathway performs in regulating immune responses and specifically antitumor immune responses. A more recent study attempted to weigh the relative role performed by different signaling pathways using blocking antibody as a therapeutic strategy. Mouse tumor cells were classified on the basis of B7-H1 expression, transplanted into syngeneic mice, and treated with agonistic anti-CD137 (4-1BB) antibody, a costimulatory molecule known to enhance the antitumor immune response [65]. Interestingly, anti-CD137 antibody treatment failed to induce tumor regression in tumor cells displaying B7-H1 but did so in tumor cells without B7-H1. The addition of anti-B7-H1-blocking antibody to anti-CD137 treatment relieved the inhibition, resulting in tumor clearance. B7-H1 on the tumors inhibited tumor lysis under extended periods of incubation, although the CTLs were functionally normal. The blockade of B7-H1 or PD-1 could relieve this inhibition. These results indicate an overarching umbrella effect conveyed by B7-H1 on tumor cells, shielding them from select immune assault under conditions of strong activation.

16.3


The CD137 protein (also designated as 4-1BB), a member of the tumor necrosis factor/receptor (TNF/TNFR) superfamily is a 30-kDa protein initially identified by a modified differential screening process [66]. The murine form is located on chromosome 4; consists of 256 amino acids; and contains a signal peptide sequence, a cysteine-rich region, a region containing multiple serines and threonines, a hydrophobic transmembrane domain, and a cytoplasmic domain. The human analog was subsequently identified from activated human T cell leukemia virus type I–transformed human T lymphocytes and was found to be located on chromosome 1 [67,68]. It is a 27-kDa protein consisting of 255 amino acids containing extracellular, hydrophobic transmembrane, and intracellular domains and showing 60% amino acid identity to the murine form [69]. The cytoplasmic domain contains five regions of sequence conservation between mice and humans, implicating these regions in CD137 function [70]. CD137 was found to be expressed as both a monomer and a dimer under nonreducing conditions [71]. CD137 mRNA was identified in unstimulated peripheral blood T cells, and on stimulation with phytohemagglutinin (PHA), phorbol myristate acetate (PMA), and ionomycin was expressed in the T cell lines, CEM and Jurkat [69,72]. In addition, B lymphocytes stimulated with PMA and cell surface Ig antibodies as well as blood

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monocytes stimulated with LPS, IL-1β, and PMA induced gene expression [72]. In mouse splenocytes, CD137 mRNA expression appears approximately 3 h after stimulation and surface expression peaks around 60 h [70,71]. In human T cells, CD137 mRNA expression was detected at 1.5 h, peaked at 8 h, and declined to baseline levels by 48 h. Under unstimulated conditions, CD137 mRNA expression was not detected in murine nonlymphoid cells. However, upon IL-1β stimulation, human CD137 could be detected in hepatoma and epithelial cells. FACS analysis showed evidence of CD137 expression on activated CD4+ and CD8+ T cells as well as B lymphocytes. These results indicate that CD137 expression occurs promptly after lymphocyte activation and its presence on activated T and B lymphocytes supported a role for the molecule in modulating immune responses. Subsequent to these findings, CD137 was found expressed on murine natural killer (NK) cells and dendritic cells (DC) [73–75]. Activation of bone marrow (BM)-derived DCs through CD137 resulted in increased production of IL-6 and IL-12 and enhanced dendritic cell stimulation of T cell proliferation [76]. CD137 expression was also identified on CD4+ CD25+ regulatory T cells [77]. Human CD137 expression was separately identified on follicular DCs, blood neutrophils, monocytes, and a significant proportion of blood vessels in malignant tissues [75–80]. Interestingly, a soluble form of CD137, generated by differential splicing in activated lymphocytes, was isolated in the sera of patients with rheumatoid arthritis, although the functional significance of this remains unclear [81]. To identify binding partners for CD137, a CD137/Fc fusion protein construct was generated, culminating in the identification of the CD137 ligand (CD137L) in an EL4 lymphoma cell line. Screening of an esophageal carcinoma (EC1) cDNA expression library led to cloning of the CD137L gene [82]. Murine CD137L maps to chromosome 17 and is composed of a 309–amino acid type II membrane protein. The human homolog was subsequently identified on activated CD4+ T cells also using a CD137/Fc fusion protein chimera. It contains 254 amino acids and exhibits 36% amino acid identified with the murine form and maps to chromosome 19. In a separate study, a fusion construct was created utilizing the extracellular domain of CD137 linked to alkaline phosphatase and was shown to bind preferentially to primary B cells and B cell lines [69]. CD137L is expressed predominantly on professional antigen-presenting cells (APCs) including dendritic cells, macrophages, and B cells [68,71,74,82,83]. However, the ligand’s expression has been identified on tumor cells and activated T lymphocytes as well [84]. CD137L has been isolated in a soluble form in patients with various hematological malignancies, including myelodysplastic syndrome (MDS), non-Hodgkin’s lymphoma (NHL), and acute myeloblastic leukemia (AML) [85]. Soluble CD137L, which was also identified in the sera of LPS-treated BM-derived DCs [76], was increased on activation of leukocytes and was capable of activating T cells through the increased production of both IL-2 and IFNγ, suggesting a potential functional role in vivo. CD137-deficient mice were generated by homologous recombination and exhibited no overt phenotypic defects [86]. Myeloid progenitors were found to be increased in bone marrow, spleen, and blood and underwent rapid turnover, indicating a role for CD137 in regulating BM progenitor development. These mice


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had decreased IgG2a and IgG3 production. However, overall neutralizing antibodies were similar, and the mice could produce specific antibodies. Interestingly, anti-CD3 antibody or mitogen-induced cell proliferation was assayed and found to be increased in CD137-deficient mice compared to wild-type. Despite the proliferative enhancement, CTL activity was reduced and production of IL-4, IL-2, and IFNγ was decreased in CD137 knockout mice. Considering the association between the activation of T cells and inducible expression of CD137, along with the expression of CD137L on professional APCs, a possible role for the CD137:CD137L pathway in modifying the immune response was investigated. Utilizing the fusion protein construct CD137:Fc, a soluble competitor to CD137, proliferation was partly inhibited when splenocytes were activated with anti-CD3. An optimal blocking effect was observed between 48 and 72 h [87]. In addition to decreased proliferation, IL-2 production was reduced by 70%. They [87] were able to show that proliferative responses of CD4+ T cells could be blocked through the use of the soluble fusion chimera in the presence of a CD137L-expressing lymphoma cell line, indicating a role for CD137 signaling in influencing both cellular and humoral immune responses. Another study demonstrated a role for the CD137/CD137L pathway in CD8+ T cell activation [88]. Monoclonal antibodies to CD137 that were costimulatory in the presence of anti-CD3 stimulation were isolated and demonstrated stimulation not only of CD4+ T cell proliferation, but also enhancement of CD8+ T cell proliferation and IFNγ production at lower anti-CD3 concentrations. Their results also indicated that activation through the CD137:CD137L pathway was biased at costimulating CD8+ T cells, in contrast to CD28, which was apparently more effective at enhancing CD4+ T cell proliferation. Agonistic anti-CD137 mAbs also enhanced CTL activity and enhanced cardiac allograft rejection, indicating a possible therapeutic target for preventing graft rejection. One of the first studies attempting to establish an in vivo role for CD137 utilized staphylococcal enterotoxin A (SEA), a superantigen that binds to MHC class II molecules on APCs [89]. SEA activates T cells bearing the Vβ3 portion of the TCR. Mice receiving both SEA and agonistic CD137 antibody resulted in reduced depletion of superantigen-activated T cells, indicating an important role for CD137 in enhancing T cell survival. The aforementioned studies focused on the role of CD137 at the priming phase of the T cell response. A subsequent analysis illustrated an important role for CD137 in restimulation [90]. By blocking previously primed splenic T cells with CTLA4:Ig fusion construct, which binds B7 receptor and thereby blocks B7-mediated costimualtory signals, CD137 was sufficiently capable of inducing a proliferative response suggesting an independent role for CD137 in secondary T cell activation. Furthermore, anti-CD137 was shown to inhibit activation-induced cell death (AICD) and prevent DNA fragmentation in reactivated T cells, a signal of apoptotic cell death. In support of a role for CD137 in T cell survival, CD8+ T cells stimulated with anti-CD3 and agonistic CD137 antibody resulted in enhanced survival and increased expression of bcl-xL and bfl-1, antiapoptotic genes considered critical mediators of cell survival [91–93]. These results support an important role for CD137 in maintaining the survival of T cells on repeated activation.

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With evidence of a role for CD137 in modulating immune responses and previous studies on the role of CD28 in costimulation, experiments were performed to address the relative impact of each on activation and memory responses. Insight into the roles of CD137 and CD28 came from studies utilizing CD137Land CD28-deficient mice [94]. CD28-deficient mice showed significantly impaired early responses to influenza virus infection as evidenced by a lower proportion of tetramer-positive CD8+ T cells and fewer IFNγ-positive CD8+ T cells on days 5 and 7 postinfection. Between 21 and 38 days postinfection, tetramer-positive CD8+ T cells in CD137L-deficient mice drop significantly in comparison to WT mice, indicating that CD137L plays an important role in maintaining the size of the memory population, while apparently having little to no role in initial expansion. CD137L-deficient mice had a slight defect at 21 days postinfection in CTL activity as compared to WT mice. With regard to the secondary response, CD137L-deficient mice showed defects in CD8+ T cells, in percentages of both tetramer-positive and IFNγ-positive cells. CD4+ T cell and antibody responses to secondary responses appeared normal in relation to WT mice, suggesting that the defects seen in CD137L-deficient mice are focused on its capacity to influence survival and/or responsiveness by CD8+ T cells. CD4+ and CD8+ T cells express CD137 during antigen-specific primary responses [95]. CD137L was capable of enhancing CD28-positive and CD28-negative antigen-specific T cell proliferation as well as increased cytokine production. Both responses were inhibited when soluble CD137:AP was added. Target cell killing was improved in CD137L-expressing, antigen-specific EL4 cells. Responses of purified CD4+ and CD8+ T cells differed according to whether they were stimulated by agonistic anti-CD137 antibody or CD137 L. CD4+ T cells released IL-2 and IL-4, with little IFNγ in response to CD137L-expressing APC. CD8+ T cells produced no IL-4, little IL-2, and more IFNγ than did CD4+ T cells. Interestingly, both CD4+ and CD8+ T cells exposed to anti-CD3 and anti-CD28 antibody stimulation resulted in increased early proliferation compared to anti-CD3 antibody and CD137L stimulation. However, over longer periods, the effect was comparable. These results suggest that CD4+ and CD8+ T cells are affected by CD137 activation, at both the priming phase and effector phases. One explanation for why the priming response is stronger to CD28 may be due to its constitutive expression. Inducible CD137 may be more important in sustaining the cell populations. These results were comparable in a separate study involving human T cells except that CD8+ T cells exhibited diminished expansion attributed to reduced IL-2 production [96]. The presence of CD137 on activated T cells stimulated further interest in understanding the costimulatory molecule’s role in influencing B cell responses. Administration of agonistic CD137 antibody within one week to mice immunized with T-cell-dependent antigens resulted in poor antibody-specific responses [97]. This result could not be replicated in humoral responses that do not require T cell–B cell interactions. CD8+ T cells were shown not to mediate this inhibition. It is also unknown whether direct binding of antibody to B-cell-associated CD137 is required for this effect. Whether this regulation is due to CD4+ T cells or CD4/CD8


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double-negative T cells was not ascertained. To further clarify the role of CD137L in vivo with the humoral response, a transgenic mouse expressing the CD137L under the control of the MHC class II I-Eα promoter was generated, producing exclusive expression of this gene on APCs [98]. Grossly, these mice displayed evidence of splenomegaly, with no significant changes observed within the thymus or lymph nodes. Despite the presence of splenomegaly, the mice showed no consistent differences in total immune cell numbers in comparison to nontransgenic littermates. However, aged mice (4–12 months old) displayed undetectable levels of B cells in both the spleen and lymph nodes with no B cell follicles present, and low levels of total serum IgG both at baseline and after immunization. Aged transgenic mice also showed poor capacity to stimulate proliferation and alloreactivity against the EL4 target cell line. These results indicate that activation of CD137 can negatively impact humoral immune responses while CD137L expression on APCs negatively influences the humoral immune response over time through reduced survival of B cells. Initial therapeutic strategies involved the use of an agonistic monoclonal antibody to CD137 in a tumor transplant system [99]. The use of anti-CD137 antibody against the poorly immunogenic Ag104A sarcoma cell line, which were well established in syngeneic mice, resulted in tumor regression as compared to control antibody administration. Mice that showed evidence of tumor clearance were rechallenged with subcutaneous tumor at 3 months and exhibited longlasting specific immunity to Ag104A. Tumor clearance via anti-CD137 antibody treatment was shown to be dependent largely on CD8+ T cells as depletion of CD8+ T cells resulted in tumor progression. Specific CTL activity was enhanced with anti-CD137 mAb treatment. These results indicate that basal activation of tumor-specific CD4+ and CD8+ T cells occurs, enabling CD137 augmentation of the antitumor response and subsequent clearance of tumor. Using a gene therapy approach, cell-bound single-chain Fv fragments from the agonistic anti-CD137 antibody (1D8) were expressed in the poorly immunogenic, low-expressing MHC class I K1735 melanoma cell line [100]. Mice inoculated with transfected tumor showed clear tumor regression as compared to K1735 cells alone. Depletion of either CD4+ T cells or NK cells resulted in tumor expansion of transfected K175 cells, supporting a role for these cell types in this specific antitumor response. Specific recall response could be induced as a result of rechallenge of mice with wild-type (WT) K1735 tumor. Importantly, mice given WT tumor and subsequently incoluated with 1D8-K1735 exhibited tumor regression, indicating this method as a potential vaccine strategy. Splenocytes from 1D8-K1735 mice exhibited increased proliferation as well as increased fraction of IFNγ-producing cells. These results indicate that this specific type of gene therapy approach may be useful as a vaccine strategy in those tumors that are poorly immunogenic and lack robust expression of MHC class I, enhancing tumor regression through Th1 CD4+ T cells and NK cells. Evidence indicating an important role for CD137 in enhancing the antitumor immune response led to further experiments to dissect the role of specific cell types that might directly or indirectly mediate this effect. DBA/2 mice were injected

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with anti-asialoGM1 antiserum to selectively deplete NK cells in vivo and subsequently inoculated with P815 tumor cells followed by agonistic anti-CD137 antibody [73]. In comparison to mice treated with control antibody, NK-cell-depleted mice displayed progressive tumor growth. Since anti-asialoGM1 antiserum could potentially deplete T cell populations, anti-NK1.1 antibody was administered in DBA/2 × C57BL/6 mice along with tumor and control or anti-CD137 antibody. This experiment confirmed the previous one, indicating an important role for NK cells in mediating the antitumor effect. However, a subsequent experiment showed that in T-cell-deficient mice, the NK cell effect was abrogated, suggesting that NK cells may be necessary, but not sufficient, for this response. Furthermore, agonistic anti-CD137-antibody-treated NK cells could not lyse P815 cells and did not result in increased IFNγ production. These results implicate NK cells in the antitumor effect, although its role may be immunoregulatory rather than by direct effector function. Considering the role that the CD137L pathway performs in modulating immune responses, studies were carried out to assess the presence and functionality of this molecule in tumor cells. FACS analysis performed on cancer cell lines, tumor cells inoculated into athymic mice, and tumor cells derived from patient tumors revealed the presence of the CD137L through the use of a CD137:Ig fusion protein construct as well as anti-CD137L antiserum [84]. Treatment of cancer cells expressing CD137L with the CD137:Ig fusion construct resulted in IL-8 production (a cytokine considered important in chemoattraction and the acute immune response), indicating that CD137L on tumors is functional. Subsequently, the group was able to show that CD137L expression on cancer cells could signal back to T cells activated with anti-CD3, resulting in IFNγ production. This response was markedly reduced by the use of a neutralizing anti-CD137 antibody. Their work identifies CD137L at the tumor cell surface and implicates CD137:CD137L in bidirectional immune responses, which could potentiate T cell activation and function. In an effort to understand and enhance DC-mediated tumor immune responses, a strategy utilizing tumor-associated antigen (TAA) delivery via replication-deficient adenoviral vector and CD137L was adopted [101]. In this study, the human papillomavirus-16 E7 oncoprotein was recombined into the adenovirus, and infected DCs were shown to direct both effector and memory CTL responses secreting IFNγ. Both responses were enhanced when CD137L was coinfected. This augmentation was revealed only at low levels of antigen availability unless another costimulatory molecule was added. Furthermore, their results indicated that the presence of CD137L resulted in a reduction of nonspecific T cell activation and induced increased expression of B7-1 and B7-2 on dendritic cells. These results indicate that CD137L expression can enhance selective immune responses where low antigen availability exists, supporting previous evidence of the CD137:CD137L pathway in enhancing immune responses to suboptimal T cell activation. In addition, they support expression of CD137L in DC-based vaccination strategies as they help generate a more favorable specific response, reducing the probability of inducing nonspecific responses to intrinsic antigens that could result in autoimmunity.


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In a subsequent study incorporating a colorectal tumor hepatic metastasis model, gene therapy of replication-defective adenovirus with IL-12 and CD137L was utilized [102]. Previous reports documented an enhanced tumor clearance with the use of intratumoral injection of adenovirus-mediated IL-12 in a colorectal tumor hepatic metastasis model [103]. However, this treatment strategy resulted in only 25% of mice surviving beyond 70 days. The protection provided was attributed to both NK and T lymphocytes [104]. Poorly immunogenic MCA26 tumor cells were injected into the left lateral lobe of the liver, allowed to mature, and then injected with various adenovirus vectors. The combinatorial administration of IL-12 and CD137L resulted in statistically significant long-term survival. This survival advantage was lost with the depletion of NK cells or CD8+ T cells. More importantly, memory responses were generated as evidenced by protection against subcutaneous tumor challenge. These results further support the role of CD137L interactions in mediating enhanced primary and memory antitumor responses, and corroborate previous studies illustrating an important role for NK cells and CD8+ T cells in the antitumor effect. A subsequent experiment utilized the same metastatic model; however, this time agonistic antibody to CD137 was administered, replacing CD137L gene transfer [105]. In addition to showing a potent tumor regression response with the administration of both IL-12 and anti-CD137 Ab, the dose of IL-12 could be substantially reduced with the same effect. The treatment was effective in both liver and lung metastasis. Once again, both NK and T cells were implicated in tumor regression as depletion of either resulted in reduced target cell lysis. Both cell subtypes were also shown to be instrumental in mediating protective effects on subcutaneous rechallenge. In a study associating the role of CD137 with CD4+ T cell help in modulating CD8+ CTL responses, antigen-specific CD4+ T cells were shown to be important in directly supporting the expansion and lytic activity of CTL [106]. Moreover, this response was enhanced by costimulatory molecules such as CD137 and abrogated by antibodies to CD137. CD8+ T cells cultured with irradiated CD4+ T cells execute in vitro antitumor immune responses, lysing melanoma cells. Their results suggest a direct cell–cell communication between CD4+ and CD8+ T cells through CD137 in productive CTL responses and subsequent tumor immunity. To further dissect the roles of CD28 and CD137L in primary and recall responses to tumor cells, A20 B cell lymphoma cells were transfected with either B7-2 or CD137L and transferred to Balb/c mice [107]. Mice that were able to reject tumor were also capable of rejecting secondary systemic challenge of parental tumor. The same experiments were then performed in mice deficient in CD28 [108]. Mice with A20 expressing CD137L were able to reject primary tumor inoculation, although mice transferred with A20 expressing B7-2 had progressive tumor growth. However, secondary challenge of mice that rejected the CD137L-transfected A20 was unsuccessful. Recovered splenocytes showed reduced CTL activity against A20 with reduced IL-2. Their results indicate that the tumors expressing CD137L can induce a priming immune response even in the absence of CD28. However, recall responses are attenuated in the absence of CD28, possibly as a result of poor IL-2 production and weakened CTL.

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Other studies have focused on the combined strengths of CD28 and CD137 in mounting a strong antitumor immune response [109]. In an adoptive therapy strategy, mice were inoculated with poorly immunogenic A9P melanoma cells and draining lymph node T cells were isolated and stimulated with IL-2 and combinations of anti-CD3, anti-CD28, and anti-CD137. Mice receiving T cells stimulated with all three antibodies showed improved long-term survival. This phenomenon could be attributed to neither changes in the CD4+ or CD8+ T cell populations nor differential expression of IL-2, IL-4, or IFNγ, all previously shown to be influenced to some extent by CD137 interactions. In another study, tumor-draining lymph node (TDLN) T cells were shown to upregulate the expression of CD137 by 70% [110]. The activation of TDLN T cells by both agonistic anti-CD137 antibody in addition to anti-CD3 and anti-CD28 resulted in enhanced T cell proliferation with reduced AICD as evidenced by less propidium iodide and annexin V staining. Cytokine production was skewed toward a Th1 phenotype with increased IFNγ and GM-CSF production and decreased IL-10 production in vitro. Transfer of anti-CD137, anti-CD28, and anti-CD3 stimulated TDLN T cells into mice receiving tumor cells resulted in less metastatic pulmonary nodules and improved survival as compared to TDLN T cells that were stimulated with anti-CD28 and anti-CD3 only. Neutralization of IFNγ abrogated the antitumor immune response, while anti-IL-10 improved tumor regression. These results support an enhancement of the CD3/CD28 antitumor immune response by CD137 as a result of augmented Th1 promotion and improved T cell survival. In another study investigating the roles of CD28 and CD137, P815 tumor cells transfected with CD137L and inoculated into syngeneic mice resulted in substantial tumor rejection [111]. However, transfection of the poorly immunogenic sarcoma cell line AG104A with CD137L or CD28 resulted in no rejection. The cotransfection of both genes resulted in tumor clearance in both the primary and secondary phases. In vivo depletion of CD8+ T cells indicated that this cell type is critical in mediating CD137L-expressing P815 antitumor immune response. These results support a synergistic effect mediated by both CD137 and CD28 in promoting antitumor immunity. Utilizing a murine squamous cell carcinoma (NRS1) cell line that expresses high levels of the costimulatory molecule B7-1, CD137L was shown to be required to induce tumor rejection [112]. These results suggest that the presence of B7-1 may not be sufficient to generate adequate tumor immune responses and directed therapy at CD137 signaling might augment the antitumor effect. The previous reports strongly support the significance of incorporating CD3, CD28, and CD137 in generating effective antitumor immune responses. Armed with this knowledge, an ex vivo strategy was developed to optimally expand CTL for in vivo use [113]. An artificial APC (aAPC) was engineered utilizing a K562 erythromyeloid cell line (which lacks HLA proteins and therefore reduces allogeneic responses) stably expressing CD137L and CD32, a human low-affinity Fcγ receptor. CD32 was used as a substrate for coating anti-CD3 and anti-CD28 antibodies on the cell surface, bringing into close proximity CD3, CD28, and CD137 L. aAPCs incorporating CD137L, anti-CD3, and anti-CD28 promoted enhanced long-term

CONCLUSIONS

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proliferation and survival of CD8+ T cells. Antigen-specific T cells could be propagated effectively using this technique and displayed specific cytolytic activity. The presence of CD137L appeared to enhance the capacity for preservation of the diverse T cell repertoire that was initially provided to the aAPC system. Their results further a synergistic role for CD3, CD28, and CD137 in mediating potent antitumor immune responses and add this system to the armamentarium of adoptive therapeutic strategies focused on antitumor immune responses. Another study investigated the role of both the anatomic site of tumor inoculation and the immunogenicity of the tumor with antitumor immune responses [114]. Mice given intracranial MCA205 sarcoma cells and GL261 glioma cells and provided with agonistic CD137 antibody manifested prolongation of survival and cure of disease in some mice with the development of systemic and memory responses. However, only a partially therapeutic effect was seen in mice given tumors subcutaneously or intravenously. No beneficial effect was seen when the tumor was a weakly immunogenic B16/D5 melanoma tumor. Mice that were able to clear tumor revealed a requirement for both CD4+ and CD8+ T cells. Interestingly, the administration of anti-CD137 during lymph node sensitization by tumor, while stimulating hypertrophy of the nodes, did not result in enhancement of the antitumor immune response, but rather had an adverse effect. In addition, adoptive transfer of TDLN cells in the presence of CD137 mAbs resulted in poor antitumor reactivity. The results of this study demonstrate that the level of tumor immunogenicity as well as the site of tumor inoculation influence the effectiveness of CD137 to deliver meaningful therapeutic effect, although the mechanisms related to these context-dependent effects remain unclear.

16.4

CONCLUSIONS

Despite advances in our understanding of the immune system during the past two centuries, vaccination and immunoprophylaxis continue to be at the forefront of immunotherapeutic strategies directed at the control and eradication of disease. While some forms of cancer will be treated from a prophylactic standpoint, a significant proportion of patients will still present with well-established tumor burden. Surgery, chemotherapy, and radiotherapy can provide some level of tumor control. However, none can potentially provide the level of selectivity and potency that could be harnessed by the immune system. Costimulatory molecules, such as PD-1/B7-H1/B7-DC and CD137:CD137L, provide attractive alternatives to anti-CTLA-4 antibody therapy in the antitumor armamentarium. Each has been shown in various model systems to influence and enhance productive antitumor immune responses. The evidence indicating that the immune system “sees” tumor and responds by the presence of tumor-infiltrating lymphocytes indicates that tumor cells develop strategies to tune down immune cells and induce tolerance, possibly exhausting T cells from their intended goal. One of the mechanisms utilized is most undoubtedly the expression of costimulatory/coinhibitory molecules that provide a

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shielding effect on the tumor and the tumor microenvironment. Immunotherapeutic strategies aimed at manipulating the PD-1/B7-H1/B7-DC pathway have the capacity to not only rejuvenate T cells but also push T cells into an activated state when other forms of “help” cannot be provided. Furthermore, signaling systems such as the CD137:CD137L pathway may improve the antitumor response by augmenting not only selective responses but also responses in low-antigen-level states. However, all of these potential future benefits must be weighed against previous evidence of deleterious effects from immunotherapy, including autoimmunity and cellular depletion. Finally, immunotherapeutic strategies will have to be tailored to the type of tumor, the level of immunogenicity of the tumor, as well as the location of the tumor, as each of these factors will most likely have significant bearing on the clinical benefit. Nevertheless, there continues to be much to gain from manipulating “signal 2” in the hopes of eradicating cancer.

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PART V DEFINING EFFECTIVE CLINICAL RESPONSES

17 ADVANCES IN IMMUNE MONITORING STRATEGIES FOR CANCER VACCINES AND IMMUNOTHERAPY Seunghee Kim-Schulze, Moshe Ornstein, and Howard L. Kaufman

17.1

INTRODUCTION

The discipline of immune monitoring for cancer vaccine and immunotherapy trials is experiencing an era of unparalleled growth because of an increased understanding of the fundamental basis of human cancer and host–tumor interactions. Knowledge gained in basic cancer immunology and molecular biology has helped identify a growing list of tumor antigens. The ability to identify specific HLA-restricted peptide epitopes within these antigens has led to development of highly targeted cancer vaccines [1–5]. The availability of such epitopes has also allowed a more sophisticated approach to immune monitoring, although the antigenic epitopes responsible for tumor recognition and rejection are not always known. Nonetheless, careful monitoring of clinical vaccine trials can provide important clues to the mechanism of a particular vaccine or immunotherapy strategy. Furthermore, as monitoring assays become more sensitive, they may be able to serve as surrogates for clinical response or for comparing the potency of different vaccines. Thus, developing Cancer Vaccines and Tumor Immunity Edited by Rimas Orentas, James W. Hodge, and Bryon D. Johnson Copyright © 2008 John Wiley & Sons, Inc. 271

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IMMUNE MONITORING STRATEGIES FOR CANCER VACCINES AND IMMUNOTHERAPY

standardized methods and validating individual assays is a major priority for tumor immunologists. This chapter focuses on current state of the art immune monitoring techniques for cancer vaccine and immunotherapy trials. The essentials of commonly used in vitro and in vivo assays are described with pertinent examples of how these assays have been used in monitoring clinical vaccine trials. Despite several attempts to standardize these assays, there is currently no universal agreement as to which assays and validation criteria should be used, and limited data exist on how particular vaccines mediate tumor regression in humans [6]. The challenge to develop a set of universal standard criteria is further complicated by the need to use ex vivo assays to determine in vivo status of the immune response, especially across a range of tumor types and stages. The future of this field will likely depend on refinements to the current set of assays and continued monitoring of clinical trials. These trials should be scientifically sound and include plans for appropriate statistical analysis of the data.

17.2

CURRENT STATUS OF TUMOR IMMUNOTHERAPY

Current immunotherapy approaches include adoptive transfer of T cells, peptides, recombinant viruses, autologous or allogeneic tumor cells, and dendritic cell (DC)based vaccines [7]. All of these strategies have shown therapeutic responses and evidence of systemic immunity in preclinical murine models and in selected patients enrolled on early-phase clinical trials [8–11]. There is emerging evidence that measurement of immune responses can be used to predict clinical responses with these agents. In studies using recombinant poxviruses expressing T cell costimulatory molecules with or without tumor antigens, objective clinical response and disease stabilization have been associated with an increase in antigen-specific T cell precursors detected by ELISpot assay [8]. Adoptive T cell immunotherapy has shown great promise, especially after nonmyeloablative but lymphodepleting chemotherapy, with objective clinical responses in 50% of patients [12,13]. Adoptively transferred T cell clones persisted in responding patients and preferentially localized to tumor sites and mediated antigen-specific immune responses that could be detected by ex vivo T cell analysis [14]. Perhaps the best example is the correlation of anti–human papilloma virus (HPV) antibody titers in women receiving an HPV virus-like particle (VLP) vaccine with prevention of cervical cancer [15,16]. These examples suggest that immune monitoring might be useful in predicting clinical responses for cancer patients receiving immunotherapy. Despite intense efforts to increase the frequency of tumor antigen-specific T cells by various vaccines, a strict correlation has not always been seen and clinical responses are rare [17]. Careful monitoring, however, has provided critical insight into the reasons for the low responses in some trials [19,20]. For example, in cancers such as ovarian and melanoma, the presence of CD4+ CD25+ regulatory T (Treg) cells in tumor is a predictor of poor patient survival [18]. These Tregs have also been shown to inhibit T cell responses following vaccine treatment in


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murine studies [20]. The role of Tregs in blocking effective antitumor immunity was based on careful immune monitoring and has led to proposals for combining vaccines with Treg blockade to improve the therapeutic activity of tumor vaccines [19]. Furthermore, combining immunotherapy with chemotherapy, radiation, and/or antiangiogenic therapy are currently being actively pursued in the clinic the basis of on immune monitoring data supporting an improved response with specific combinations of these strategies [20,21]. While the future of immunotherapy for the treatment of cancer is promising, it is difficult at present to directly compare the various immunotherapeutic approaches because monitoring assays have not been standardized with respect to technical or analytic methodology. Most of the currently used assays have focused on monitoring adaptive immune responses, such as antibody titers and T cells, although innate immune responses may also be important in immunosurveillance and therapeutic responses in cancer patients. Nonetheless, the focus on T cell immunity is probably important in terms of data from preclinical studies and analyses of the tumor microenvironment in cancer patients. The ideal T cell assay should be sensitive, specific, reliable, and reproducible. The assay should be easy to perform and reflect the status of the immune response in vivo. In addition, the assay should demonstrate a close correlation with clinical outcomes. The assays currently used to measure immune response can be divided into those assays that are performed directly in patients and those that are performed in vitro using patient-derived blood or tissue samples. Examples of direct in vivo assays are the delayed-type hypersensitivity response, imaging of lymph nodes and tumors, and whole-body scans. The more commonly employed in vitro T cell assays include the enzyme-linked immunospot assay (ELISpot), tetramer assay, intracellular cytokine flow cytometry (CFC), quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), microarray/proteomics, proliferation assay, and standard cytotoxicity assay. Humoral immunity is typically determined by enzyme-linked immunosorbant assay (ELISA). The choice of an assay for monitoring of tumor vaccines needs must be based on a complete understanding of the characteristics and limitations of the assay, the proposed mechanism of action for the vaccine or regimen under investigation, the availability of pretreatment and posttreatment samples, and the skill of the immunologists doing the assays. Immune Monitoring Assays A large number of tests are available for immune monitoring of cancer vaccine and immunotherapy trials. The primary objective of this monitoring is to determine the effects of a vaccine or regimen on priming or augmenting specific humoral or cellular immune responses. The prevalence and functional characteristics of tumor-specific B and T cell responses can be quantitated and monitored in vitro by various methods as summarized in Table 17.1. While ELISA has become the standard for detecting antibody responses, there are many assays for monitoring T cell responses. Although all of the T cell assays in Table 17.1 are currently in use, three assays—ELISpot, tetramer staining, and CFC—have emerged as

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T A B L E 17.1. Characteristics of In Vitro Immune Monitoring Assays Assay Methods

Target

Detection Limit

Quantification

Sensitivity

Assay Parameters

ELISpot Tetramer analysis CFC

Cellular Cellular

+++ ++

+++ +++

+++ ++

Cellular

++

+++

++

qRT-PCR

Molecular (DNA/RNA) Molecular Molecular Cellular Cellular

+++

+++

+++

Functional Nonfunctional, phenotypic Functional, phenotypic Nonfunctional

+ N/A N/A N/A

++ + N/A N/A

+ + N/A N/A

Nonfunctional Nonfunctional Functional Functional

Molecular (protein)

+++

+++

+++

Nonfunctional

Microarray Proteomics Proliferation Cytotoxic assay ELISA

a Abbreviations: ELISpot—enzyme-linked immunospot; CFC—cytokine flow cytometry; qRT-PCR—quantitative reverse transcriptase polymerase chain reaction; ELISA—enzyme-linked immunosorbant assay; N/A—not applicable; the number of + signs depicts the level.

first-line methods for monitoring T cell responses in most clinical trials. These assays are relatively reliable and sensitive and allow ex vivo T cell analysis at the single-cell level. In addition to these standard assays, other approaches, such as flow cytometry, proliferation, and cytotoxicity assays may provide additional functional information. Other more complex methods, such as quantitative real-time PCR, microarray analysis, and proteomics have been useful for gene and protein profiling of the tumor microenvironment and as complements to first-line monitoring assays [22]. More recently, efforts at in vivo imaging have been evaluated, and these may provide additional and complementary information to the more familiar in vitro assays. Enzyme-Linked Immunospot Assay (ELISpot) The interferon-γ (INFγ) ELISpot assay is a commonly used procedure and has been extensively validated for the detection and quantification of functional T cell responses in patients treated with cancer vaccines. ELISpot is an attractive and powerful assay that allows tracking of antigen-specific T cells in the peripheral blood of patients who receive vaccines provided enough T cells are available for pre- and postimmunization sampling. ELISpot also permits quantitative measurement of cytokine secreting cells at the single-cell level directly ex vivo or after in vitro stimulation, which can be used to expand low-frequency T cells. The function of cells can be inferred from the types of cytokines secreted by cells in response to antigenic stimuli. ELISpot assay is often the first choice in vaccine evaluation


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since high-throughput screening using a microtiter plate format is efficient and can accommodate large numbers of samples and multiple antigens. In general, the ELISpot assay has the lowest limit of detection and can often detect as few as 1 in 100,000 T cells [23]. The ELISpot assay is conducted by pretreating a nitrocellulose-coated 96-well microtiter plate with an anticytokine antibody. While INFγ is commonly used, as its secretion represents a functionally active T cell, other cytokines can be used as well. The antibody-coated plates are then used to plate a defined number of T cells derived from peripheral blood lymphocytes, usually through Ficoll gradient separation. The T cells are activated by coculture with autologous or artificial antigen-presenting cells (APCs) pulsed with a peptide or other antigenic material. While autologous DCs are ideal for this purpose, they are not always available. Alternative sources of APC for ELISpot assay include allogeneic or xenogeneic APC; human lymphoid hybrid T2 cells, artificial HLA-A2 dimers, and human chronic myelogenous leukemia K562 cells transfected with the HLA-specific genes [24]. On activation of the T cell, cytokines are released and captured by the antibody on the nitrocellulose membrane. The cells are washed away and a secondary cytokine-specific antibody is added to the plate and a color reaction linked to the secondary antibody will result in a “spot” at the site of a functional T cell. The number of spots can be counted in each well, and because the number of T cells plated is known, the precursor frequency of the tumor-reactive T cells in vivo can be estimated. The assay can be used to run cryopreserved pre- and posttreatment samples reducing the interassay variability. For situations where the T cell frequency may be very low, T cells can be expanded by repeated exposure to antigen in a process known as in vitro stimulation (IVS). While IVS may help detect low frequencies of reactive T cells, prolonged IVS may bias the response and not reflect the actual in vivo responses. The ELISpot can be performed with bulk T cells or on separated populations to define individual CD4+ and CD8+ responses, provided that MHC class II and class I antigens are available. The cell specificity can also be determined by repeating the assay in the presence of class I and class II blocking antibodies. Although the ELISpot assay is often most sensitive when using a single peptide for stimulation, the assay can be used to define T cell reactivity against multiple peptides or even whole-cell lysates. This may be advantageous in situations where the peptides or antigens are not defined. There has been a concerted effort to validate the ELISpot assay in order to achieve superior reliability and reproducibility of performance [25]. While the definition of a positive immune response for the ELISpot assay varies with the affinity and avidity of the antigen(s) being tested and the inter- and intraassay variability, there is general consensus on how to define a positive immune responses by ELISpot. Most laboratories use a two- to threefold increase in spot-forming units (SPU) for T cells exposed to an experimental antigen compared to nonantigen or irrelevant antigen-stimulated T cells as a positive response [26,27]. An additional criterion is an increase in T cell frequency at least 2 standard deviations above the mean frequency [23,28]. For specific antigens, one or both of these parameters may be used and the assay usually requires validation for individual antigens using a

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normal pool of donor T cells to establish baseline levels of response. Many clinical tumor vaccine trials have been monitored by ELISpot assay, and evidence for a correlation between positive ELISpot data and clinical responses has been reported [45–53]. In addition to cytokines, which define one functional aspect of T cell activation, other markers that more closely represent the cytotoxicity profile of activated T cells have been used to detect functional T cells in vaccinated patients. Shafer-Weaver and colleagues reported that T cell reactivity in a granzyme B ELISpot assay was more closely associated with functional status of cytotoxic T cells than in other assays, including an INFγ ELISpot [29]. Other functional readouts for ELISpot assays have included IL-10, IL-4, IL-5, GM-CSF, and perforin. This highlights an important advantage of ELISpot assays in providing both a quantitative assessment of T cell precursor frequency and functional data on the quality of the response. The major limitations of the ELISpot assay include the need for sufficient numbers of cells, the skill of the technician, and the inter/intraassay variability. Viable T cells can be obtained from fresh and frozen peripheral blood lymphocytes. When pretreatment and posttreatment samples are evaluated at the same time, there is a reduction in the interassay variability. In previous studies, the variability of ELISpot assays due to blood processing was determined by testing leukopheresed PBMC shipped overnight in medium versus cryopreserved PBMC from matched donors. There were no major differences in ELISpot results in fresh or cryopreserved T cells, suggesting that with proper handling in cryopreservation does not reduce the sensitivity of the assay [30]. Intraassay variability can be reduced by technical training of lab personnel and routine validation on normal donor samples to establish reproducibility and reliability data. Tetramer Analyses Tetramer analysis is a flow-cytometry-based technique that replaces monoclonal antibodies with folded peptide–MHC complexes attached to a color substrate. Initially four complexes were used (hence the term tetramer), and the tetramer binds to the cognate T cell receptor because of the specificity provided by the peptide. Today, other complexes, including dimers and pentamers, have all shown promise in the identification of monoclonal T cells. The assay can be used to quantitate the number of antigen-specific T cells in samples with as few as 10,000 cells. Thus, the availability of sufficient cells for the assay is rarely a problem for tetramer analysis. The interaction of T cell receptors on T lymphocytes with tetrameric MHC–peptide complexes in vitro mimics the specific interaction of T cells with APCs in vivo. Tetramers consist of four biotinylated HLA–peptide epitope complexes bound to streptavidin conjugated with a fluorescent dye. The nature of the multivalent binding increases the affinity and enables staining and analysis of cognate T cells with the sensitivity of detection as low as 0.02% of the total T cell pool (i.e., approximately 1 in 50,000 lymphocytes) [28,30]. In contrast to many other methods, such as the ELISpot assay and the 51 Cr-release assay, analysis by tetramers does not depend on the functionality of the T cells, that is, the secretion of cytokines on activation


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and cytolysis of targets. Thus, a major disadvantage of tetramer analysis is that all T cells recognizing the specific antigenic peptide embedded in the proper MHC complex will be labeled and recognized regardless of functional status [28,30,31]. Currently, tetramer staining in combination with costaining by antibodies against other cell surface or intracellular cytokines has become a frequent practice in order to add functional information to the assay. This combination enables the direct assessment of antigen-specific T cells for lineage differentiation, activation status, and cytokine production in a single assay, and thus offers the advantages of speed and the ability to identify subsets of reactive cells. There are several other advantages to the tetramer assay. Once monoclonal T cells are identified by tetramer analysis, tetramer-labeled T cells can be cloned by cell sorting for further characterization and study. The tetramer approach not only is useful for identifying T cells in peripheral blood but can also be used to identify T cells in tissue through in situ hybridization. For example, combinational tetramer analysis of CD3+ CD8+ tetramer+ cells from tumor, lymph node, and PBMC obtained from HLA-A2+ patients with head and neck cancers was used to define the frequency and functional status of the T cells [32]. The CD3+ CD8+ tetramer+ cell frequency was significantly higher in tumor-infiltrating lymphocyte (TIL) than autologous PBMC. Furthermore, the CD4+ CD25+ TIL population was enriched with cells expressing Foxp3, GITR, and CTLA4, hallmarks of Treg cells. Finally, CD8+ TIL had low TCR zeta-chain expression and produced little IFNγ after ex vivo stimulation indicating the functional anergy of these cells in the suppressive tumor microenvironment [32]. This study nicely demonstrates the wealth of information that can be attained with tetramer analysis. One limitation is the need for properly folded tetramers, which may not always be available, and the need to have defined antigens as targets for analysis. Tetramer analysis can also be used to monitor the number and activity of adoptively transferred T cells. In a clinical trial of adoptively transferred CD8+ T cell clones with specificity for MART1 and gp100 in patients with metastatic melanoma, tetramer analysis indicated that persistence of transferred T cell clones in vivo correlated with positive clinical outcomes [14,33,34]. There are, however, limitations with tetramer assays since functional information is not inherent in the assay. For example, in a trial evaluating p53 peptide-specific CD8+ T cells in the circulation of patients with squamous cell carcinoma of the head and neck (SCCHN), there was no correlation between clinical response and p53-specific T cell accumulation in the tumor. These results highlight limitations of this assay, in addition to the complexity of interactions between the tumor and the host immune system [35]. Another limitation of tetramer assays is the need for different complexes for each distinct HLA allele and peptide, rendering it difficult to apply to rare HLA haplotypes and across large outbred populations. The reagents for tetramer assays are also relatively expensive and require expertise in flow cytometry for data acquisition and analysis. Nonetheless, advances in the biochemistry of tetramer formation and advances in flow cytometric analyses have made the technique highly popular for monitoring immunotherapy trials. Similar to all assays, ongoing validation is

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helpful in reducing variability and increasing the sensitivity and specificity of the assay. Flow Cytometry Multiparametric characterization of cytokine production on a single-cell basis with high throughput of samples is an attractive feature of flow cytometric analysis. The technique allows simultaneous use of four or more antibodies and is thus particularly useful for the small subsets of lymphocytes modulated by immunotherapy. Cytokines are particularly good candidates for multiplexed analysis, although numerous other functional molecules can be used. Through intricate networks and complex feedback mechanisms, cytokines modulate each other as well as a multitude of cellular events and play a critical role in activation of immune responses after cancer vaccination. Historically, the quantity and breadth of tumor antigen-specific CD8+ T cell responses have not always correlated with clinical outcome. Thus, it is critical to develop immune monitoring assays that can assess the quality of the tumor antigen-specific CD8+ T cell response by measuring multiple CD8+ T-cell-specific functions simultaneously. Flow cytometry requires monoclonal antibodies against specific molecules of interest, such as cytokines or other functional proteins. The assay can be used to stain cell surface markers, which can be used to identify subpopulations of cells, for example, CD4+ versus CD8+ T cells. The assay can also be used to define intracellular molecular targets, such as IFNγ or CTLA4, by permeabilizing the cells. The technique is powerful and requires few cells but cannot define individual T cell clones. It can, however, be combined with tetramer analysis to identify individual clones, or with ELISpot to provide another measure of cytokine-producing cell frequency. As refinements in technology have been developed, this technique has been applied to a variety of tissue samples with excellent results. As in other assays, validation and standardization is critical before these assays can be used in clinical immune monitoring. Newer flow-cytometry-based approaches have allowed for the simultaneous analysis of five or more cytokines as well as the expression of memory-defining markers on tumor antigen-specific T cells. This analysis finely dissects the immune response and enables identifications of functional subsets of cells not previously known. Multiparameter flow cytometry standardization is important since the tumor-specific CD8+ T cell response in vivo would be defined by many criteria such as homing, expansion, and effector functions of individual cells [36]. A good example of the utility of multiparameter analysis can be found in a trial monitoring quantitative and qualitative immune response in PBMC of 41 bladder cancer patients [37]. The study revealed that percentages of cells expressing CD3, CD4, and CD8, or CD56 were found to be significantly lower in patients than healthy controls. The mean percentage of cells expressing CD4 was significantly lower in patients showing recurrence as compared to patients without recurrent disease. The percentages of CD4+ T cells producing IFNγ, IL-2, and TNFα were significantly reduced, while the mean levels of IL-4, IL-6, and IL-10 in patient T cells were significantly higher as compared to healthy controls. Patients with bladder cancer


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appear to develop a TH 2-type response, which is associated with disease progression. The lymphocyte evaluation along with cytokine measurement thus provides a sensitive and valuable tool for evaluating the function of cell-mediated immunity in these patients and can also find application in therapeutic monitoring of bladder cancer patients as new targets for immunotherapy are discovered [37]. Similar to other methods, flow cytometry analysis has its own advantages and disadvantages. The technique can be used to evaluate other molecular players in the immune response, such as perforin, granyzmes, and costimulatory molecules. Cells with similar surface phenotype but different cytokine production profiles, and cells exhibiting different functional characteristics can be differentiated using this technique [38]. Caution must be observed, however, for cross-reactivity of multiple cytokine antibodies, and interpretation may be difficult at times because of changes in the phenotype of individual cells undergoing differentiation. The assay allows a functional readout without a restriction for MHC haplotype as in the tetramer assay, and the response can be read for a broad set of T cells simultaneously in a relatively short period of time. Like other assays that measure functional features, the variation between assays is always a major concern, making proper standardization imperative. Quantitative Real-Time RT-PCR The polymerase chain reaction (PCR) revolutionized the field of molecular biology and allowed for accurate detection of gene transcripts from minute volumes of genetic material. The technique is based on an amplification process that occurs when small probes complementary to known DNA sequences within a gene are amplified during DNA replication in vitro. The technique can be used to directly quantitate the level of gene transcripts within a sample through a technique called quantitative real-time PCR (qRT-PCR). This method uses a fluorescent dye attached to the specific gene probe, which will be diluted with each round of replication. Thus, the degree of fluorescence dilution will correlate with the number of gene transcripts present in the samples. In contrast to the cellular immune assays discussed previously, the major advantage of qRT-PCR is in the accurate measurement of relevant gene expression levels within target tissues, such as T cell populations or the tumor microenvironment [39,40]. The assay has been most useful in evaluating the tumor microenvironment, where there are quantitative or qualitative differences in immune modulating molecules and cells, for example, the balance between effector and regulatory T cell accumulation in tumors. The qRT-PCR assay may also be more reliable since a few studies have suggested that cellular immune assays may not be reliable for cells located within established tumors [41–43]. Quantitative real-time PCR is a powerful and rapid method to quantify gene transcripts, and it measures the buildup of fluorescence with each PCR cycle and generates quantitative fluorescence data at the earliest phases of the PCR cycle when replication fidelity is highest [44,45]. Guadagni and others demonstrated that CEA mRNA expression measured by qRT-PCR in the blood of carcinoma patients was strongly correlated with disease status [46]. The potential use of qRT-PCR

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for immune monitoring came from an early study using clone-specific primers to determine T cell composition and frequency. The technique was used to determine T cell clonal expansion, contraction, and deletion following adoptive transfer [47]. The major advantages of qRT-PCR are the high sensitivity, ability to quantify transcripts, and reproducibility of the assay. The procedure can detect as few as five molecules with excellent dynamic range and in a linear manner over several orders of magnitude. The disadvantages include the cost of instruments and reagents, the inability to currently employ high-throughput systems, and the inherent danger of false positives due to RNA carryover or contamination. The availability of RNA is also required and various techniques for RNA amplification from small samples have been proposed to identify gene expression even from minute samples [39,48,49]. Microarray Analysis Microarray technology is based on the availability of an expressed sequences tag (EST) database of the human genome and allows collection of extensive biological data. These data can be used to enhance our understanding of physiological processes and disease states by profiling tissue and single-cell populations at the RNA and protein levels. The technique is particularly useful for comparative analysis of genomewide mRNA expression patterns. In cancer research, high-throughput evaluation of genetic or protein profiles in specific tissues can be used to identify unique signatures in tumor compared to normal tissue, or in primary compared to metastatic tissue. Microarray analysis has generated significant information on tumor-associated and tumor-specific biomarkers for cancer diagnosis and as targets for therapeutic drug development [50–52]. Currently, a variety of array protocols and software are available for analysis. Originally, cDNA-based microarrays employed the most popular and widely used form of cDNA robotically printed onto glass slides and subsequently hybridized to fluorescently labeled probes to compare cellular expression patterns from different samples. Another leading technology relies on the use of oligonucleotides and has the advantages of using synthetic DNA from a large number targets rather than a limited number of cDNAs. Using a functional microarray analysis, CD8+ T cells from 10 melanoma patients after gp100 peptide vaccination were found to have a diverse set of patientand antigen-specific cytokine expression profiles, indicating differences in clinical responses [53]. For example, melanoma patients with increased secretion of IFNγ and TNFα in response to gp100-based vaccination remained free of disease recurrence, whereas patients who showed discordant secretion of IFNγ and TNFα experienced recurrence sooner [53]. Similarly, genes involved in patient survival were identified through a 17,500-cDNA microarray of 43 tumor tissues from 38 patients with stage III/IV melanoma [54]. Expression data analysis revealed that patients with increased expression of genes related to innate and acquired immunity lived longer than did patients without such expression profiles, confirming the interplay between immunologic mechanisms and the natural history


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of melanoma. Future subset analyses of the specific genes may provide important clues to how the immune system interacts with melanoma to prevent disease or respond to immunotherapy used in treatment of established disease. Evaluation of protein levels can be used to complement data obtained from microarray analysis and may provide additional information that cannot be obtained from focusing on expression patterns at the RNA level. This may relate to a poor correlation between mRNA and protein levels or to posttranslational modifications resulting in multiple protein isoforms [55]. Proteomics Analysis While PCR and microarray data provide information on gene and gene expression status, it is clear that gene expression does not always correlate with actual protein expression. The direct analysis of proteins within cells or tissue samples is referred to as proteomics analysis. This technique can provide data on all proteins in a particular tissue sample, and has been used for a variety of diagnostic and monitoring purposes. The use of proteomics for immune monitoring remains theoretical but merits mention because of the type of information that can be provided, and it represents an improvement from standard gene techniques that may be misleading regarding the in vivo environment. Much of the potential for proteomics has focused on identification of biomarkers in serum samples. Proteomics may offer a novel approach to immunologic monitoring that complements other immunologic assays. In principle, quantitative protein expression profiling is a crucial part of proteomics and requires methodology that is able to efficiently provide accurate and reproducible differential expression values for proteins in two or more biological samples. By studying the T cell activation status and levels of protein expression, it may be possible to quantitate the magnitude of immunologic responses. Proteomics could also provide a tool for obtaining valuable information regarding underlying regulatory mechanisms and pathways as elegantly demonstrated in a study of pancreatic cancer [56]. The proteomic analysis indicated that many of the differentially expressed proteins between cancer and normal healthy pancreatic tissues are involved in protein-driven interactions between ductal epithelium and extracellular matrix remodeling to promote tumor growth, migration, angiogenesis, invasion, metastasis, and immunologic escape. These findings will no doubt facilitate identification of the candidate biomarkers for early responses to therapeutic interventions. Major disadvantages of this technique include the labor- and resource-intensive costs, the lack of quantitative data, low sensitivity, and a high rate of false-positive results. The assay requires a lengthy time for sample preparation, protein quantification and identification, and extensive bioinformatics analysis, which result in long assay times. Most importantly, it has been difficult to establish automation or to perform a full proteomics analysis in large-scale clinical trials. Nonetheless, future advances in high-throughput technology and bioinformatics processing mechanisms will likely improve the use of proteomics as a tool for immune monitoring.

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T Cell Proliferation Assay Although more sophisticated techniques, such as ELISpot and flow cytometry, are typically used for monitoring immune responses, standard T cell proliferation and cytotoxicity assays can often be used to help validate the more complex assays. The T cell proliferation assay is based on the ability of T cells to undergo proliferation on antigen exposure (usually in the presence of APC), and the proliferation can be measured by standard 3 H- thymidine incorporation during DNA synthesis. The assay is relatively easy to perform but can be a lengthy procedure that requires radioactive material. The proliferative status is often reported as a stimulation index (SI), which is calculated as the ratio between cell proliferation in the presence versus absence of antigenic stimuli. The SI has proved useful in measuring immune responses to a variety of vaccines [57]. SI values, however, do not adequately reflect the proliferative capacity of low-frequency responder T cells, and repeated in vitro nonspecific stimulation can influence the outcome. In fact, functional T cell anergy in vivo may be reversed during in vitro stimulation, and thus these assays may not reflect the in vivo status of patients [58]. The utility of this assay has nonetheless shown promise in the clinical setting. For example, in a trial using a p53 cancer vaccine in small cell lung cancer patients, clinical responses to subsequent chemotherapy were closely associated with an increased proliferation of p53-specific T cells [59]. In order to circumvent the use of radioactive material and to better define T cell responses, T cell proliferation has been measured by labeling the cells with the succinimidyl ester of carboxyfluorsecein diacetate (CFSE). As the CFSE-labeled cells proliferate in response to antigenic stimulation, the dye is diluted out in the dividing cells. The dilution of CFSE can be used to determine the number of cell divisions and calculate a proliferative index. This method also allows for better characterization of proliferating T cells as the cells are analyzed by flow cytometry to assess proliferation [60,61]. T Cell Cytotoxicity Assay The potential cytotoxic activity of CD8+ T cells can be directly measured by lysis of target cells expressing the cognate antigen recognized by the T cells. MHC-expressing cells pulsed with specific peptides or intact MHC-restricted tumor cells have been used as targets in T cell cytotoxicity assays. Traditionally, target cells are loaded with 51 Cr prior to incubation with different ratios with effector cells. The cytotoxicity is determined by the amount of radioactive 51 Cr released from the target cells after in vitro culture for several hours. The percent specific lysis is calculated by dividing the total release by maximum release with spontaneous release subtracted from both. This in vitro–specific cell lysis is considered to directly reflect the potency of cytotoxic effector cell killing in vivo. For this reason, many studies of cancer vaccines designed to induce CD8+ T effector cells have attempted to demonstrate specific target cell killing in vitro using this assay. This has often been complicated in clinical trials because of difficulty in obtaining autologous tumor cells, the lack of defined antigenic peptides, the low


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sensitivity and high background of the assay, and the need for large numbers of T cells. Another drawback to this assay is the lack of information at the single-cell level, which is possible with many of the other more sophisticated T cell assays. In addition, the frequent necessity for T cell in vitro stimulation (IVS) prior to cytotoxicity assay can also distort the true magnitude of effector T cell cytotoxicity in vivo. Alternatively, analysis of the molecules released during cell death, such as lactate dehydrogenase (LDH), is replacing the traditional 51 Cr release assay. In addition, it is most likely that flow cytometric analysis at the single-cell level of antigen-specific cell lysis will replace the bulky 51 Cr release assay in the future. Advances in In Vivo Imaging Direct in vivo imaging of the intact immune system is a rapidly advancing field and could provide real-time identification of various immune components responding to vaccines or immunotherapy. While the application of direct imaging in patients is just beginning, intriguing data from murine studies have supported this as a powerful approach for following in vivo immune responses. Currently available technologies to study tumors in murine models include bioluminescent imaging (BLI), magnetic resonance imaging (MRI), computed tomography (CT), and positron emission topography (PET). The BLI method enables tracking of the cellular flow patterns by following the images produced by luminescent or fluorescent molecules that have been integrated into the genomic DNA of the cells of interest. The most common reporter genes in use today are the green fluorescent protein (GFP) and luciferase (Luc) genes [62,63]. The use of BLI to monitor cells in vivo is quickly being applied to the characterization and treatment of various diseases, including cancer. In one study, human prostate cancer cells were transfected with a Luc gene and injected into SCID mice [89]. Careful analysis of tumor cells after transplantation demonstrated increased fluorescence over 14 days as the tumor grew, indicating the ability for tumor cells to stably express luciferase and use the technique for in vivo monitoring of tumor growth [64]. BLI has also been shown to be effective in monitoring the response to treatment of intraperitoneal tumors [65]. The human cervical carcinoma line HeLa transfected with Luc was injected into the peritoneal cavity of SCID mice. BLI was able to document a detailed regression of tumor in the mice after adoptive transfer of cytokine-induced killer (CIK) T cells [65]. Hu and coworkers elegantly demonstrated the trafficking of preactivated T cells labeled with superparamagnetic iron oxide nanoparticles in tumors that were undergoing immune rejection using MRI [66]. In another study, differentially labeled CD4+ CD25+ Tregs and TH cells could be tracked in the same lymph node using two-photon laser scanning microscopy. This study found that Tregs limited TH cell access to the lymph node and decreased contact time with DCs, thereby preventing TH cell activation [67]. Whole-body imaging can also be helpful in monitoring the immune response to immunotherapy. Thorne and colleagues were able to demonstrate that Luc-expressing CIK T cells infected with vaccinia virus were effective in eradicating ovarian tumors by releasing the virus only after infiltrating the tumor

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[68]. This labeling technique of CIK cells further illustrates the potential for tracking immune cells in vivo. MRI, CT, and PET all offer more accurate three-dimensional structural imaging of tumors than BLI provided T cell clones can be appropriately labeled for detection without affecting their functional capability. The use of MRI, CT, and PET are quite expensive and require significant image collection time, which may limit their development. In contrast, BLI offers the advantage of being relatively inexpensive with short image collection times. The current application of BLI, however, is complicated by poor resolution profiles and difficulties in orienting tumor signals in a two-dimensional plane. The eventual application of in vivo imaging is a promising area, and further refinements in optics and labeling technology will likely see an expansion of this technique for real-time immune monitoring of cancer patients.

17.3

ASSAY VALIDATION

All immunologic monitoring assays should include extensive quality control and assay validation studies before use in cancer immunotherapy trials. There should be clear definition of a positive immunologic response to immunotherapeutic treatment taking into account assay imprecision. Preanalytical variability should be minimized by following standardized sample collection and processing and strict adherence to the standard operating procedures (SOP) of optimized assays. The inherent assay variability should be determined by performing assays in replicate samples (intraassay) and also by analyzing replicate samples from the same blood at different timepoints (interassay). Currently, there are joint efforts by laboratories across academics and industry to develop and optimize in vitro assays monitoring T cell function, including standardized protocols for use of all reagents, controls, and standards. The assays have to be developed for the maximum optimization of specificity and reproducibility. It is acknowledged that the best means of validating assays will be achieved through large multicenter cancer vaccine clinical trials [6]. This will allow for assay comparison among the participating laboratories in order to achieve universal standards with acceptable interlaboratory variations.

17.4

CONCLUSIONS

Each assay reviewed herein has unique advantages and disadvantages, and the selection of appropriate immune assays for monitoring clinical trials is not a trivial issue. The evaluation of cancer vaccines and immunotherapy regimens will require standardization of the selected assays and statistical analysis plans in place before an assay can be used for immune monitoring. The standardization of current monitoring assays is of high scientific priority for cancer vaccine evaluation. There are three key aspects for assay standardization and validation:

REFERENCES

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1. A set of robust assays needs to be established that are standardized and validated in both academic and industry laboratories. This requires some degree of cooperation among experts to select and develop particular assays. 2. Assays selected for patient monitoring require constant evaluation and characterization with attention to the use of standardized reagents (such as peptides, tetramers, control sera, virus panels), and these reagents need to be made available for common use with quality control measures. 3. Development of novel assays of potential value should be an ongoing effort and subject to review as they undergo translation from preclinical to clinical settings. Obviously, the ideal assay would correlate with clinical responses, and continued use of immune monitoring techniques as part of well designed clinical trials will help provide data to better define the potential effectiveness of individual assays. Assay selection may also depend on the type of vaccine or immunotherapeutic intervention. The development of reliable, sensitive, accurate, and specific in vitro or in vivo immune monitoring assays that can successfully quantify and characterize immune responses in patients has been a major goal for tumor immunologists. Such assays would be beneficial for predicting vaccine efficacy and would also provide a method for furthering our understanding of antitumor immune mechanisms. Current data and prevailing opinion support the use of multiple functional assays, such as ELISpot and CFC in combination with other quantitative assays, like tetramer staining, for immune monitoring considering that immune responses to cancer vaccines involve a complex array of cellular and soluble factors [69,70]. Development and application of quantitative molecular approaches to measure the properties of single cells and complex tissue samples is also possible using molecular profiling and bioinformatic networking programs. The direct imaging of cells and molecular components of the immune response in vivo is in the early stages of development but offers an opportunity to directly visualize the immune response in patients. These new techniques should prove to be helpful in providing more relevant immune monitoring methods for predicting clinical responses to immunotherapy and will no doubt improve our understanding of the complex interactions between tumors and the host immune system.

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18 IMMUNE EVALUATION OF CANCER VACCINES Theresa L. Whiteside

18.1

INTRODUCTION

Antitumor vaccines are delivered to cancer patients with the goal of reversing the state of immune unresponsiveness to the tumor and inducing robust tumor-specific reactivity. This strategy is based on the principle that the host’s immune system is involved in cancer development, progression, and elimination [1,2]. It has been documented that immune surveillance plays a role in protection from cancer development, and immune elimination of tumor cells involves a combination of innate and adaptive responses [3]. At the same time, the immune system may promote tumor survival by immune selection of tumor variants resistant to immune-mediated intervention [4]. In addition, tumors develop various mechanisms for evasion from the host immune system [5]. Thus, complex interactions between the host immune system and the tumor evolve and become established during cancer progression. Antitumor vaccines are designed to perturb these interactions and shift the balance in favor of the host. The most successful vaccines should be able to induce robust antitumor cellular and humoral immunity, culminating in tumor rejection and Cancer Vaccines and Tumor Immunity Edited by Rimas Orentas, James W. Hodge, and Bryon D. Johnson Copyright © 2008 John Wiley & Sons, Inc. 291

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establishment of long-term immunologic memory. To determine how successful a vaccine is in achieving these goals, it is necessary to serially evaluate changes in tumor-specific responses before, during, and following vaccine administration. Immune monitoring has become an essential component of all clinical trials testing vaccines and their antitumor efficacy. However, immune monitoring of patients with cancer represents a challenge for several reasons. First, the presence of cancer is associated with a degree of immune suppression, which may be quite considerable, if the tumor burden is large. The resulting immunologic imbalance may be difficult to measure and quantify, because responses to nontumor antigen and mitogens are normal, and only tumor-specific responses are compromised. This could reflect the existence of tolerance to self, as the majority of tumor-associated antigens are “self” [6]. It makes sense that patients who are to be enrolled in clinical protocols testing a therapeutic cancer vaccine should be evaluated for the integrity of their immune response. In patients with cancer, delayed-type hypersensitivity (DTH) responses to recall antigens are intact, except in advanced disease, and those to tumor epitopes are absent. Confounding effects of age, previous therapies, even a cancer type, which may or may not be strongly immunosuppressive, all complicate evaluation of the immune status in individual patients, so that consistent results across patient groups are a rare finding. Further difficulty arises from the fact that no established universal criteria are available for evaluation of tumor-specific immunity in candidates for cancer vaccines. Thus, the question of which patient is immunologically eligible for a vaccine remains unanswered. It would seem reasonable to assume that patients whose immune system is compromised by tumor presence are not the optimal candidates for antitumor vaccines. A general consensus has evolved, largely on the basis of experience, that vaccination after surgery or other primary therapy, in the setting of minimal residual disease (MRD) is likely to be more effective than that administered in the presence of tumor. However, no test currently exists that predicts a response to the vaccine. This creates a problem with selection of patients and selection of methods for monitoring. It further emphasizes the need for defining the criteria that would help in establishing guidelines for immune monitoring and evaluations of cancer vaccines.

18.2

TOOLS NECESSARY FOR SUPPORT OF VACCINATION THERAPIES

To be able to initiate and conduct vaccine-based therapeutic trials, a specialized laboratory, equipped and prepared to support vaccine production and postvaccine monitoring, is necessary. Services required of such a laboratory are extensive and varied (Table 18.1). The process of experimental vaccine introduction to the clinic begins with extensive preclinical evaluations. The laboratory responsible for vaccine production plays a critical role at this stage. In the case of investigator-initiated clinical trials, preclinical studies that lead to vaccine development are often done in experimental animals. The process of translating the concept and methods used in animal vaccination studies to the clinical arena may require extensive adaptation. In general, this involves reproducing vaccine generation under cGMP conditions

TOOLS NECESSARY FOR SUPPORT OF VACCINATION THERAPIES

293

T A B L E 18.1. Laboratory Services Necessary for Support of Cancer Vaccine Trials Preclinical services Adapt and translate research findings to humans Establish vaccine composition and release criteria Establish vaccine production process (for cellular vaccines) Upscale vaccine production to meet clinical requirements Vaccine preparation (cGMP) Vaccine formulation and delivery to the clinic Process human tissues to be used as an antigen source Process leukapheresis products Produce cellular vaccines Test for safety and quality Vaccine monitoring (GLP) Prevaccine testing: HLA typing, DTH tests Serial specimen collection, login, processing Cryopreservation and banking Assays on “batched” serial specimens Result interpretation QA/QC program to ensure reliability and meet regulatory requirements

with human tissues or cells and limited testing of the immunogenic potential of the vaccine in ex vivo experiments with human cells. Not all cancer vaccines require this type of preclinical investment, but cellular vaccines, such as dendritic cell (DC)-based vaccines, cannot be initiated without prior establishment and validation of the production–release process. As these steps are necessary for filing of an investigator-initiated drug (IND) application with FDA, all cellular vaccines undergo the preclinical development in a cGMP facility, which will be ultimately responsible for their production in a clinical trial. At this stage, the vaccine composition and its final characteristics (e.g., cell viability, purity, and potency) and the release criteria, which must include absence of microorganisms (vaccine sterility) and endotoxin levels (

Cancer Vaccines and Tumor Immunity

Cancer Vaccines and Tumor Immunity

Tumor Immunology and Cancer Vaccines (Cancer Treatment and Research)

Cancer Vaccines and Immunotherapy (Cambridge Cancer Series)

Cancer Genome and Tumor Microenvironment (Cancer Genetics)

Cancer Vaccines and Immunotherapy (Cambridge Cancer Series)

Cancer Genome and Tumor Microenvironment

Handbook of Cancer Vaccines

Tumor Models in Cancer Research (Cancer Drug Discovery and Development)

Tumor Angiogenesis: Basic Mechanisms and Cancer Therapy

The p53 Tumor Suppressor Pathway and Cancer

Innate and Adaptive Immunity in the Tumor Microenvironment (The Tumor Microenvironment)

Natural Killer T cells: Balancing the Regulation of Tumor Immunity (Cancer Drug Discovery and Development)

Innate and Adaptive Immunity in the Tumor Microenvironment (The Tumor Microenvironment)

Peptide-Based Cancer Vaccines (Medical Intelligence Unit)

Natural Killer T cells: Balancing the Regulation of Tumor Immunity

Origin of anti-tumor immunity failure in mammals

The Tumor Microenvironment (Cancer Drug Discovery and Development)

Tumor Prevention and Genetics III (Recent Results in Cancer Research)

Cancer Vaccines: Challenges and Opportunities in Translation (Translational Medicine)

Genomics, Proteomics and Vaccines

Tumor Angiogenesis and Microcirculation

Tumor Suppressor Genes in Human Cancer (Cancer Drug Discovery and Development)

Infection and Immunity

Aging, Immunity, and Infection

Aging, Immunity, and Infection

Mucosal Vaccines

Tumor Suppressor Genes in Human Cancer (Cancer Drug Discovery and Development)

Cytokines Stress and Immunity

Viral Pathogenesis and Immunity

Cancer Vaccines and Tumor Immunity